I have a bike with SRAM derailleurs. I can get the front derailleur to shift well to lowest and highest gears but I can't seem to get the middle chainring to be able to handle more than about 3 rear cogs without rubbing. This is driving me nuts. I've never had this occur on another system. Is there anything to try?

The same thing has been addressed just the other night. Here is a Copy & Paste for you to copy:
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Take the cable off the FD. Now install it from scratch as per Park Tool Repair:

https://www.parktool.com/repair/readhowto.asp?id=75

And BicycleTutor:

https://bicycletutor.com/adjust-front-derailer/

When you have it properly trimmed and are ready for the cable - a new one wouldn't hurt - you want to pull the cable taut - not too tight, taut. Then apply 48 to 60 inch-pounds to the pinch-bolt. Then put it through it's paces.

It's always easier and faster to install a FD from square-one, than it is to make adjustments with it already attached. This tends to fix one thing - while throwing another out of kilter. Start fresh. You'll get it.

Adding a barrel-adjuster can solve many problems with the cable coming loose - or being too tight.

OK thanks will try as soon as my new cables I ordered arrive

You didn't say which fd and what shifters you have so there might be a problem with the components matching.

True - I hope you know the difference between a top-swing, down-swing, and a both FD. As well as seat-post diameter, and if it's designed for the gears you're running.

Good luck.

I'm not sure what seat post diameter has to do with shifting or his front derailleur.

Just whether or not he can mount it on the bike. Other than that - no difference regards shifting.

I believe you meant seat tube, not seat post.

Yep! LOL. Fixed. Thanks.