Running HR is highest, then cycling, then swimming. Rule of thumb for max used to be 7bpm difference run to bike, then 10bpm down to swim. Your HR can go higher than 130 for the swim – you just need to measure it on a shorter, faster course!
Training sounds good. Try to make 1 session a week a fast one (100m repeats) to get some speed, and the others as slower endurance ones. I did 3K Max before my Iron man, no need to hit the full 3.8K your current times indicate about 1h20 for your swim
If your swim is in a lake, then get some practice in there – it has less buoyancy than the sea.
Biggest benefit I found was learning to relax for the open water swim, sounds like you are already there for this. Which race are you doing?