How do you write a input for 3 measurements so you write them in all at once and not line by line?

I'm making a really simple program for required measurements (length, width, thickness) of a package and I know there is way to input the all three without writing them line by line.

Measure = float('Specify in mm measurements length,width,thickness:')
M1 = Measure.split(',,')

K = L+W+T

if float(M[0]) 100 <= x <= 600 and float( M[1]) x <= 100 and float(M[2]) x <= 90 and K <= 900:
    print('Allowed!')
else:
    print('Not allowed!')

I get it wrong and I don't know where to go from here.