The number one thing I shout at people when they’re trying to finish a triangle is “CONTROL THE HEAD”. If your head is forced down forward, you lose the ability to generate power, and it’s far easier to get caught in the triangle.
Anytime you can, raise your head and look up when you’re caught in a triangle or a proto-triangle.
To help support your head in an elevated position (or your spine in normal thoracic extension, to use fancy jargon 
) place your ‘trapped’ hand on their hip bone, and use your free hand to support it. Push down hand, like a dip or push up. If his triangle is weak enough, you may break free, or use a ‘Salsa Shake‘. If he’s still got it on, keeping your head elevated will help any other escape.
This funky escape works even if your head is bent forward, although it requires a degree of flexibility and timing.
In the triangle, hop up onto your hunkers.
The closer your legs are to his hips, the easier it is to raise your head and the stronger you’ll be.
Fall back and with your trapped arm catch your opposite leg, i.e. if your right hand is trapped grab your left shin. Mark played around with this and found if you push your feet into his armpits before grabbing your leg, you’ve more time to escape and he’s less likely to sit up and take a mounted triangle. We didn’t photo this but it’s in the video.
Your hips are more powerful than his ability to keep his triangle here, so extend your hips. Lift up your chest, and push your feet into his chest to escape.
Easy peasy, lemon squeezy.