I am trying to figure out which Ergon grips to buy - GC3, GR2, or GR3. There are lots of positive reviews of each one in various places but not much in the way of comparison. I understand that the GC3 provides the maximum number of grips but it's not clear to me how much I would actually make use of them.

Also, I have carbon bars. Do I need to use plugs in the ends to prevent cracking when installing the Ergons? Any suggestions for what plugs to use other than the pricey ones Ergon sells itself?

i think the reason that direct comparison between the different ergon products are hard to find is because most people only have experience with the product that they eventually bought.

i bought GC3s for my hybrid last summer and i instantly fell in love with them. i've always placed a premium on multiple hand positions, so the GC3s were the natural choice for me.

as for plugs in carbon bars, that i don't know about as i have an aluminum flat bar.

the Company : https://www.ergon-bike.com/us/en/home to ask the source.

Safety precautions!
These grips are not suitable for the use with carbon handlebars. according to Ergon

The issue with carbon bars, based on various posts here an elsewhere, is that they can crack if the Ergon grips are torqued too tight. That said, a lot of folks with carbon bars report no problems. The solution is to use bar plugs that reinforce the the bars internally and prevent the cracks. Ergon makes them, but they are pricey, and I'm curious if others have used different solutions.

Alloy bars, here, GC3 on my Brompton, I use the extra length, and hook,
like I do, with my trekking bars , open palm across the opening,
fingertips on the front.

if you just lightly grip the bar just barely stopping it from rotating , maybe,

but why are you wedded to the carbon bars?
Why not shelve them and get metal ones..

Ergon is pretty specific about those reinforcing plugs - they're intended for use with thin-walled aluminum bars only. Ergon's instructions regarding thise particular grip models and carbon bars is summed up very concisely. "DON'T"

Might be woth noting that those reinforcing plugs are grouped with the Technical , ie competition oriented grips, which is where you'd expect to find thin-walled alloy bars. Carbon fiber bars has never caught on in professional mtb biking the same way it has in consumer mtb biking. Maybe because the average guy with carbon bars really only uses the bike in the street.

I went through a similar process last year debating between a GR1, GC2, GR3, GC3 (in order of hugeness of the bar end). I ended up picking the GC2 and have to say I'm very happy with it. The stubby bar ends don't look like much but they are enough for 2 fingers and do offer a surprising amount of leverage when climbing. So that might be a plug for the less obtrusive GC2 or GR2 rather than the huge bar ends on the 3 series.

Those huge bar ends are very comfortable though.

Lawyers make them say 'no' to carbon bars.
I know that the local Ergon reps use them on their carbon bars.

Yes, because many people would over-tighten them and crack their bars. Just like people cracked stays in their LHTs when mounting kickstands and demanded a new bike from Surly. That's why we have warning stickers on everything these days because people are too dumb.

If no off the peg inserts are made to go inside the CF bars you you have,
You might get a machine shop to make a solid plug to fill the inside diameter and Epoxy it in there,
perhaps even a solid rod of More Carbon fiber composite.

Are you absolutely 100% completely positive? There are a number of carbon fiber covered alloy bars on the market simply because CF sells and thats the cheapest way to market the CF look. They're pretty convincing - in fact they had one rep and a store owner fooled till I showed them the inside of the tube on the demo with a flashlight.

It seems pointless to use lightweight CF bars, and then add heavy bar ends, particularly if the manufacturer specifically recommends against this. I'd go with the suggestions of replacing the CF bars with alloy. This is a safety issue, and should not be taken lightly.