L.H.S
=2aa+b+2bb+c+2cc+a+(b−c)(c−a)(a−b)(b+c)(c+a)(a+b)
=2a(b+c)(a+c)(a+b)(b+c)(a+c)+2b(a+b)(a+c)(b+c)(a+b)(a+c)+2c(a+b)(b+c)(c+a)(a+b)(b+c)+(b−c)(c−a)(a−b)(b+c)(c+a)(a+b)
As,
2a(b+c)(a+c)(a+b)(b+c)(a+c)=2a2b+2a2c+2c2a+2abc(a+b)(b+c)(a+c)
2b(a+b)(a+c)(b+c)(a+b)(a+c)=2b2c+2ab2+2a2b+2abc(b+c)(a+b)(a+c)
2c(a+b)(b+c)(c+a)(a+b)(b+c)=2ac2+2bc2+2b2c+2abc(c+a)(a+b)(b+c)
(b−c)(c−a)(a−b)(b+c)(c+a)(a+b)=ab2+bc2+ca2−ab2−bc2−ca2(b+c)(c+a)(a+b)
upon adding all this we get,
=2a2b+2a2c+2c2a+2abc+2b2c+2ab2+2a2b+2abc+2ac2+2bc2+2b2c+2abc+ab2+bc2+ca2−ab2−bc2−ca2(b+c)(c+a)(a+b)
=6abc+3a2b+3a2c+3ab2+3ac2+3b2c+3bc2(b+c)(c+a)(a+b)
=3(2abc+a2b+a2c+ab2+ac2+b2c+bc2)(b+c)(c+a)(a+b)
=3(a2b+a2c+ab2+abc+bc2+b2c+abc+ac2)(b+c)(c+a)(a+b)
=3(a2(b+c)+ab(b+c)bc(b+c)ac(b+c))(b+c)(c+a)(a+b)
=3(a2+ab+bc+ac)(b+c)(b+c)(c+a)(a+b)
=3(b+c)(c(a+b)+a(a+b))(b+c)(c+a)(a+b)
3(b+c)(c+a)(a+b)(b+c)(c+a)(a+b)
=3
R.H.S