Sebelumnya baca tutorial : LATIHAN FLUTTER WEEK#1 : COLUMN CLASS

Ubah bagian body menjadi

body: new Center(
  child: new Row(
    children: <Widget>[
      Container(
        margin: EdgeInsets.all(5.0),
        width: 100.0,height: 100.0,color: Colors.orange,
        child: new Center(child: new Icon(Icons.camera,color: Colors.white,size: 80.0,),),
      ),
      Container(
        margin: EdgeInsets.all(5.0),
        width: 100.0,height: 100.0,color: Colors.blueAccent,
        child: new Center(child: new Icon(Icons.image,color: Colors.white,size: 80.0,),),
      ),
      Container(
        margin: EdgeInsets.all(5.0),
        width: 100.0,height: 100.0,color: Colors.redAccent,
        child: new Center(child: new Icon(Icons.favorite,color: Colors.white,size: 80.0,),),
      )
    ],
  )
)

Semoga bermanfaat