Generally speaking, improvement occurs when you are able to practice well. Practicing should not be easy (it may or may not be enjoyable) and it should cause you to eventually become fatigued (mentally or physically). You should think about the mechanicals of what you are doing, each time you do a repetition and evaluate your proficiency of execution and the results you achieve.
If you can stay mentally/physically sharp and focus on each shot for 150 rounds, I don't see any reason why you can't do 150 rounds in a practice session. I also don't think it matters if you split it into 3 sessions - unless you can't maintain focus for the full 150 rounds in which case you should split it into the shorter sessions. There's a difference between shooting for fun and practicing. There's nothing wrong with going to shoot just for enjoyment, however, just pulling the trigger for an hour isn't an hour of practice.
It's a lot like working on your golf game. You can go to the driving range and hit 200 balls a week, but if you don't think about what you're doing and you're just swinging away, you're not really practicing - you're just hitting golf balls. Practice should be difficult because the goal is to work on things you're not good at to improve them.
So I guess my answer to your question is - choose the practice regimen that fits you best. If you're able to bear down and focus for 150 shots in a session, great, do that - it will cost you less in range fees. If you're not able to maintain good focus for that length of time, split it up into smaller sessions.