You are given hydrogen (1), oxygen (8) and krypton (36) and your targets are elements 39 to 41. Clearly the krypton and hydrogen are useful, but what about the oxygen?
Instead of disposing the oxygen, use five oxygens to make zirconium (5 * 8 = 40) and use the spare hydrogen and krypton to make yttrium (36 + 3) and niobium (36 + 5). This is a good idea because it relieves you from having to use krypton as a base for fusing zirconium, which would mean the krypton would be an ingredient in three outputs. Reactors only have two outputs.
Split up the water in to hydrogen and oxygen with some neat drive-by unbonding.
To fuse many molecules together in this instance, you'll need to spam many input and fuse commands. Using exclusive waldos will be helpful. One waldo should take krypton and fuse it up to yttrium then sync to allow the other waldo to act. That waldo should either output the yttrium or fuse two additional hydrogen to make niobium and output that.
Stuck on making zirconium out of oxygen? Try this
Okay, here's how to make and split up the other outputs.
There is also a challenge to complete the level with only two reactors. All you have to do is merge the water to oxygen part of the first reactor with the zirconium producing reactor.