My wife and I have budgeted $300 for each of us for Christmas.  I've been shooting 3-gun for a few years.  Below is a list of what I use.  Suggestions on things I didn't know exist but obviously need would be greatly appreciated.

AR- PSA 16 carbine with a Troy delta battle rail topped with a Vortex sparc.  All else is stock.  I use magpul magazines with nordic comp extensions.

Shotgun- Rem 1100 w/9 round tube and aftermarket plastic furniture and dave's easy loader.  I use a taccom 8up dual loader.

Pistol- Glock 34 w/ ghost rocket 3.5 kit and warren tactical sights.  I've got a couple dawson mag extenstions. I have a couple safariland mag pouches that I like very much.

I use a instructor's belt right now and have put in a request for a CR speed belt kit.

Thanks in advance guys!

Are you running a comp/brake on your rifle? If not, I would start there.

Do you have the baby stroller (i.e. gun carrying device)?

$300 will not get you far, but have you thought about going to a scope vs a red dot? Depending on the distances at your local matches you shoot, you may never need it.

I use my kids' radio flyer to haul my junk around.  I will look into the compensator.  It will need to be one that gives off minimal concussion as this rifle doubles as part of our safety plan.  I know a couple of good studies have been done comparing muzzle devices.  Thanks!

Do all the guns run 100% reliably (particularly the shotgun)? If the shotgun gives you trouble, consider selling it and upgrading to a Benelli, Beretta or VersaMax. If the guns all run well, spend ~$100-150 on a shot timer, then the rest on practice ammo. This will likely yield far more improvement than buying more equipment. If you must buy equipment, my guess would be that more load 2/4 shotshell caddies would give you the biggest bang for your buck.

My 1100 has run 100% for I'd say going on at least 10 matches.  I keep it clean and lightly lubed as well as replacing the o-ring often.  Thanks VERY much for the suggestion of a shot timer.
We have a separate line item in our budget for buying ammo, so we're good there.

Do you have a variety of rifle magazines?   A 20 rd magazine, a 40 rd magazine and possibly a coupled pair of 30's might come in handy.

If you don't have one and have the time to practice, a .22LR conversion for your AR would make a nice present.

How about an upgrade to your shooting glasses or ear muffs.  I went to electronic muffs a few years ago and couldn't believe I waited so long.

Another consideration is get a nice set of airsoft pistols.  Good ones seem to hold together and you can get one to match your competition handguns.  When my grandkids are in town its time for a little competition in the back yard.  Decent practice and a lot of fun.

Think about some of the matches you and the wife went to.  Was there anything that you felt you needed to be a better competitor?  Put that at the top of your list.

Before you get too carried away, also keep in mind, it not the arrow, its the Indian.  Hope this helps.

Thanks very much for the responses.  I shoot with some very well funded 3-gunners.    "Indian, not the arrow" is something I have to repeat every other second or so.

I tell myself that ........... a lot.

Thanks for the recommendation!