Quoted:
so which one should I get for a midlength H, H2, or H3 ? mine does not have any markings, so I would assume it is not of the H type. Doesn't colt use an H type?
If your buffer doesn't have markings and it weights 3 oz. per the chart then it's a standard carbine buffer.
You could try out your buffer if it's a standard type, it might work, or you might want a slower bolt velocity for more reliability, you could use a heavier buffer like the H or H2, etc.
New Colt models now and usually the law enforcement models with carbine gas systems comes with the H buffers, mine was an older spare from my former Colt carbines which I tried and it works.
Also I'm using a Colt M16 BCG on my midlength carbine, I've been using it with my standard buffer for over three years now without any problems (YMMV).
Even with the carbine gas systems that I had been using for more than 23 years now, I was lucky and never had any problems using the standard carbine buffers.