Nogi triangle escapes!
As with most submissions there is a point of no return in triangle chokes (unless you have some rampaging power to lift up the submission guy and KO-slam him to the ground). So the "escape" really is an avoidance to the final lock up, because once it's locked up properly you have no space to move, with no movement possible there cannot be any escape and then it's either tap or lights out in 3-5secs.
So if it's not yet locked up, but your opponent is close to do so (you're between his legs, on of his legs is already above your shoulder and he is just about to sink his leg) most important thing is to stay cool headed! You want to have your elbow (from the arm which is also between his legs) tight to yourself (so he can't easily manipulate your arm) on his belly/hip/groin (on the side of your arm), so your upper arm blocks the deep sinking and you keep some space to work with. What you want to prevent is that he crosses your arm over to the other side, you may support yourself with having your hand against his opposite inner thigh. Then in the same time you want to step up with your leg of your free arm side (giving more pressure to your elbow) and then move your body in the same direction as the arm which is between his legs (so if your right arm is between his legs you move to your right side) trying to step with your knee of the same side on his belly. In that position you may be able to break his leg triangle open or at least to create enough space to slip out. Or with your arms either you can slip out your trapped arm followed by your head or sneak your free arm in and at least advance into his guard and from there to work on.
Hope that this can give you an idea on what to do.