Watching a lot of 3 gun videos, I see that many shooters put the support hand way out on the end of the handguard.
Why is this?
I've tried it, and it's not very comfortable. I've done a lot of offhand shooting over the years (highpower rifle silhouette) and it just feels more comfortable for me to pull my hand all the way back on the handguard, or even to grip the magwell.
I'm sure there's a reason for this. Such as target to target transition. But that's the only thing I can think of.
I'm modifying my AR for 3 gun. I purchased an angled fore grip. I'm trying figure out the best place to put in on handguard.

It does help you drive to targets faster, but use what grip you like. I use a verticle grip on my rail, but don't grab it, I use it as an index point and have my hand 1/2 on the rail, and 1/2 on the grip. Kind of a hybrid grip. It's what works for me and is most comfertable.

It helps drive the gun, but everyone has to find the right position for them and their particular physique. I'd suggest you get out to the range and try a few transition drills... let the timer tell you what hand position works best for you, rather than relying on how things feel.

Also, 3-gunners with a vertical foregrip or AFG are definitely in the minority - most prefer an uncluttered handguard for more flexibility when shooting off props. Again, don't be afraid to experiment, but let the timer be your guide.

I recently saw a training video where one pro (Forgot his name) explained that he extends his arm out for better stability.  He went on to explain that if you had a 10 foot barrel, you would have better stability if you could have someone hold up the barrel at the end rather than a foot or two out.  I guess same goes with a bipod for long distance bolt gun shooters, even though Jerry Miculek prefers to have his bipod closer to the magazine for 3 gun.  Like as what was said, whatever you feel more comfortable doing but look at what the people who have been doing this a while are doing.  They probably have experimented with a lot of what we have or are trying already.

Driving the rifle, and with your thumb riding along the top of the rail, I would imagine some muzzle-flip control as well.

I like running a short vertical grip, I go half on the grip half on the rail. I use it to index and I like to use it to drive the grip into a barricade so I get a more steady platform.