You define
# row
Div([
Label(),
Div([Dropdown()], width='33%') # column
Label(),
Div([Dropdown()], width='33%') # column
Label(),
Div([Dropdown()], width='33%') # column
])
# row
Div([
Div([Slide()], width='33%') # column
Div([Slide()], width='33%') # column
Div([Slide()], width='33%') # column
])
But I propose
# row
Div([
Div([Label(),Dropdown(),Label(),Slide()], width='33%') # column
Div([Label(),Dropdown(),Label(),Slide()], width='33%') # column
Div([Label(),Dropdown(),Label(),Slide()], width='33%') # column
])
or at least
# row
Div([
Div([Label(),Dropdown()], width='33%') # column
Div([Label(),Dropdown()], width='33%') # column
Div([Label(),Dropdown()], width='33%') # column
])
# row
Div([
Div([Label(),Slide()], width='33%') # column
Div([Label(),Slide()], width='33%') # column
Div([Label(),Slide()], width='33%') # column
])
Minimal working code.
I removed className="three columns" to remove gaps between columns and I used width="33.33%" to better use width.

import dash
import dash_core_components as dcc
import dash_html_components as html
import plotly.express as px
import pandas as pd
external_stylesheets = ['https://codepen.io/chriddyp/pen/bWLwgP.css']
app = dash.Dash(__name__, external_stylesheets=external_stylesheets)
app.layout = html.Div(
html.Div(className='row', children=[
html.Div(children=[
html.Label(['Dataset:'], style={'font-weight': 'bold', "text-align": "center"}),
dcc.Dropdown(
id='dropdown-dataset',
options=[
{'label': 'Diabetes', 'value': 'diabetes'},
{'label': 'Boston Housing', 'value': 'boston'},
{'label': 'Sine Curve', 'value': 'sin'}
],
value='diabetes',
searchable=False,
clearable=False,
),
html.Label('Slider', style={'font-weight': 'bold', "text-align": "center"}),
dcc.Slider(
min=0,
max=9,
marks={i: 'Label {}'.format(i) if i == 1 else str(i) for i in range(1, 6)},
value=5,
),
], style=dict(width='33.33%')),
html.Div(children=[
html.Label(['Model Type'], style={'font-weight': 'bold', "text-align": "center"}),
dcc.Dropdown(
id='dropdown-select-model',
options=[
{'label': 'Linear Regression', 'value': 'linear'},
{'label': 'Lasso', 'value': 'lasso'},
{'label': 'Ridge', 'value': 'ridge'},
{'label': 'Polynomial', 'value': 'polynomial'},
{'label': 'elastic-net', 'value': 'elastic-net'},
],
value='linear',
searchable=False,
clearable=False
),
html.Label('Slider', style={'font-weight': 'bold', "text-align": "center"}),
dcc.Slider(
min=0,
max=9,
marks={i: 'Label {}'.format(i) if i == 1 else str(i) for i in range(1, 6)},
value=5,
),
],style=dict(width='33.33%')),
html.Div(children=[
html.Label(['Add data'], style={'font-weight': 'bold', "text-align": "center"}),
dcc.Dropdown(
id='dropdown-custom-selection',
options=[
{'label': 'Add Training Data', 'value': 'training'},
{'label': 'Add Test Data', 'value': 'test'},
{'label': 'Remove Data point', 'value': 'remove'},
],
value='training',
clearable=False,
searchable=False
),
html.Label('Slider', style={'font-weight': 'bold', "text-align": "center"}),
dcc.Slider(
min=0,
max=9,
marks={i: 'Label {}'.format(i) if i == 1 else str(i) for i in range(1, 6)},
value=5,
),
],style=dict(width='33.33%')),
],style=dict(display='flex')),
)
if __name__ == '__main__':
app.run_server(debug=True, port=8080)
CSS file used in code threads full width as 12 columns (similar to other CSS frameworks - ie. Bootstrap) so if you want to create 3 columns with gaps then you should use nameClass="four columns" which means "four of 12 columns" and 4/12 gives width 1/3 - and then you don't have to use style=dict(width='33.33%')

import dash
import dash_core_components as dcc
import dash_html_components as html
import plotly.express as px
import pandas as pd
external_stylesheets = ['https://codepen.io/chriddyp/pen/bWLwgP.css']
app = dash.Dash(__name__, external_stylesheets=external_stylesheets)
app.layout = html.Div(
html.Div(className='row', children=[
html.Div(className="four columns", children=[
html.Label(['Dataset:'], style={'font-weight': 'bold', "text-align": "center"}),
dcc.Dropdown(
id='dropdown-dataset',
options=[
{'label': 'Diabetes', 'value': 'diabetes'},
{'label': 'Boston Housing', 'value': 'boston'},
{'label': 'Sine Curve', 'value': 'sin'}
],
value='diabetes',
searchable=False,
clearable=False,
),
html.Label('Slider', style={'font-weight': 'bold', "text-align": "center"}),
dcc.Slider(
min=0,
max=9,
marks={i: 'Label {}'.format(i) if i == 1 else str(i) for i in range(1, 6)},
value=5,
),
]),
html.Div(className="four columns", children=[
html.Label(['Model Type'], style={'font-weight': 'bold', "text-align": "center"}),
dcc.Dropdown(
id='dropdown-select-model',
options=[
{'label': 'Linear Regression', 'value': 'linear'},
{'label': 'Lasso', 'value': 'lasso'},
{'label': 'Ridge', 'value': 'ridge'},
{'label': 'Polynomial', 'value': 'polynomial'},
{'label': 'elastic-net', 'value': 'elastic-net'},
],
value='linear',
searchable=False,
clearable=False
),
html.Label('Slider', style={'font-weight': 'bold', "text-align": "center"}),
dcc.Slider(
min=0,
max=9,
marks={i: 'Label {}'.format(i) if i == 1 else str(i) for i in range(1, 6)},
value=5,
),
]),
html.Div(className="four columns", children=[
html.Label(['Add data'], style={'font-weight': 'bold', "text-align": "center"}),
dcc.Dropdown(
id='dropdown-custom-selection',
options=[
{'label': 'Add Training Data', 'value': 'training'},
{'label': 'Add Test Data', 'value': 'test'},
{'label': 'Remove Data point', 'value': 'remove'},
],
value='training',
clearable=False,
searchable=False
),
html.Label('Slider', style={'font-weight': 'bold', "text-align": "center"}),
dcc.Slider(
min=0,
max=9,
marks={i: 'Label {}'.format(i) if i == 1 else str(i) for i in range(1, 6)},
value=5,
),
]),
],style=dict(display='flex')),
)
if __name__ == '__main__':
app.run_server(debug=True, port=8080)
EDIT:
Of course you can also organize it in separated rows (if it helps you)
# row
Div([
Div([Label()], width='33%') # column
Div([Label()], width='33%') # column
Div([Label()], width='33%') # column
])
# row
Div([
Div([Dropdown()], width='33%') # column
Div([Dropdown()], width='33%') # column
Div([Dropdown()], width='33%') # column
])
# row
Div([
Div([Label()], width='33%') # column
Div([Label()], width='33%') # column
Div([Label()], width='33%') # column
])
# row
Div([
Div([Slide()], width='33%') # column
Div([Slide()], width='33%') # column
Div([Slide()], width='33%') # column
])