It is enough to prove the inequality for a+b+c=2.
Let us denote x=ab+bc+ac, then
a2+b2+c2=(a+b+c)2−2(ab+bc+ac)=4−2(ab+bc+ac)=2(2−x).
From the obvious inequality x(2−x)≤1, we get
2+2abc≥2≥2x(2−x)=(ab+bc+ac)(a2+b2+c2)==ab(a2+b2)+cb(c2+b2)+ac(a2+c2)+abc(a+b+c),
Using the equality 2abc=abc(a+b+c), we arrive at the conclusion.
Equality occurs, for example, a=b=1, c=0.