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It seems defining the variables as variables would solve the problem.

f1,f2,f3,a,b,c,d = var('f1,f2,f3,a,b,c,d');
eq1 = f1 == a + b + f2;
eq2 = 2*f2 == c + d + f1;
eq3 = f3 == f1 + f2;
solve([eq1,eq2,eq3],f1,f2,f3);

This would give the answer @cacl314 mentioned