Also, can you determine how they are plumbed? Are the two weak ones on their own branch leading from the pump, or does it look like they might be at the end of a return line?

Water tends to take the path of least resistance, so if you have two branches of three jets each - it wouldn't be surprising to see the jets on the end of each line to have lower outputs than the ones which are closer to the pump side of things.

I know in our pool I have three return jets. Two of them are on a Y shaped return line (each at their own end of the Y) and one is on its own return. The single return jet has noticeably higher output than the two on the Y.