I have three matched 16MB 30-pin simms that look broadly similar and a fourth that make up the set, but which is of a different size and make. They work together without any problem.

I think a properly matched set is always preferable, but given that you have the three, you could do what I did and try finding any one 16MB 30-pin simm at the same speed (70ns) and see if that did the trick. If it did work, it'd be a good deal cheaper than four. If it didn't, well, you are not much farther behind than when you started....