=CHOOSE(COUNT($B4:$D4)+1, "underdefined", "underdefined", IF(B4<>"", B4, C4*D4), "overdefined")

The formula says if the number of inputs is 0 or 1, the problem is underdefined, and if it's 3, it's overdefined.

If exactly 2 inputs are provided, then if the voltage is specified, copy the voltage; if not, compute the voltage as the product of current and resistance.

All the formulas have exactly the same form, and differ only in the FALSE part of the IF function.