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I don't know what I is in your code, perhaps you meant I1. It looks like to me that you have one equation involving I1 and you want to substitute the values of IR1 and IR2 from the second two equations into it. If that's really all you want, then calling solve is overkill. Try this:

sage: eqn = I1 == IR1 + IR2
sage: eqn = eqn.subs(IR1 == U1/R1)
sage: eqn = eqn.subs(IR2 == U1/R2)
sage: eqn
I1 == U1/R1 + U1/R2
sage: eqn.rhs()
U1/R1 + U1/R2
sage: eqn.rhs().factor()
(R1 + R2)*U1/(R1*R2)