3D effect at bottom of container in Flutter

That isn't anything 3D. It can easily be achieved by using the boxShadow property of decoration of the Container widget.

You can then play around with things like color, blurRadius to get the desired effect.

Sample code:

class Shadow extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Shadow')),
      body: Container(
        color: Colors.black,
        child: Center(
          child: Container(
            width: 300,
            height: 300,
            decoration: BoxDecoration(
              color: Colors.blue,
              borderRadius: BorderRadius.circular(40),
              boxShadow: [
                BoxShadow(
                  color: Colors.blue.withOpacity(0.5),
                  offset: Offset(0, 25),
                  blurRadius: 3,
                  spreadRadius: -10)
              ],
            ),
          ),
        ),
      ),
    );
  }
}

Output