My 2000 S10 started miss,ing, tracked it to number 3 cyl, replaced injector, now number 1 cyl is missing

It goes away because the rpm is increased and smooths out the shaking. Not because the problem is going away. Does the check engine light flash? When was it last tuned up?

Yes, the check engine light flashes

Do you have spark at cylinder 3

Yes, I have spark at number three cylinder,

I'm assuming you tested cylinder 1 injector with an ohm meter and that's how you knew it was bad. Its not uncommon for two or three injectors to fail with each other. There was one engine that due to design was notorious for the injectors getting hot on just one side. When one injector failed it was recommended to replace all of them on that side because they only had a few months left if they were lucky. I can't for the life of me recall what vehicle it was. If cyl 3 is getting spark then it is either missing fuel or compression so either a new fuel injector or you will have to figure out where its losing compression with a leak down.

what is the mileage? perhaps new coils are in order...