The obvious first step in extending the total session length is foreplay. Foreplay should really be the focus of any sexual encounter, especially in a threesome. There are some women that are really into penetration, but I don’t think I’ve ever heard a woman complain "when my boyfriend and I have sex there is just too much foreplay!". Foreplay primes the pumps and makes achieving orgasm much easier.

Condoms can be useful for slowing down a man’s excitement, and are a good idea for group sex situations anyways.

Some guys can go multiple times a night, but others are definitely "one and done". If you can go multiple times without too much refractory period there is no shame in going once, taking a break and focusing on foreplay, and going again. Some of my best nights were marathon sessions with several such breaks.

In closing, assuming because the women in question are sisters there was little bisexual activity between them; this really cramps a lot of options for making sure everyone is satisfied. Most successful threesomes involve some girl-girl action while the guy takes a breather. Without that option threesomes become much trickier, and it’s much more difficult for both women to feel fully satisfied.

Anyways I’am surprised that you lasted that long. Having sex with two beautiful women. My suggestion is that you pleasure them with hands and mouth, and don’t let ANYTHING touch your penis. Do this for about 20 minutes or until they both or one of them cums. Then you can use your penis. If it works out you’ll at least have beaten you old record. My other suggestion is too watch a lot of porn and test yourself to see how long you can last. Remember practice makes perfect.