All of the following triangles involve first isolating the inside arm. Another way to think about this is like having a near-side overhook, completely taking the first arm out of the way. This first method for controlling the inside arm involves a simple hip-switch maneuver: as you switch your base, your right knee can slide underneath your partner's right arm, thus clearing their elbow from inside your hip (and jacking it up onto your hips momentarily). From there, you can lift their shoulder or tricep (I prefer the shoulder for better control), then switch your hips back while maintaining upward pressure on their inside arm. Voila—you have control of the arm now.

If your partner is underhooking with their free left arm, you'll just need to pin the wrist to their stomach. Now switch your hips, encouraging them to want to try to bridge up and get on top of you. When they make the initial effort to bridge, it's helpful to try to push them back flat with your right foot, lifting under their thigh, almost like a butterfly hook from the outside. On the second effort, they'll often overcompensate and jump right into the triangle. You can keep the wrist through the entire transition until you've caught the triangle, ensuring a solid shot at the submission before sacrificing top position.