I'll be flying an international flight and since my carry-on bag is big and my laptop bag doesn't have much space, I'm thinking of bringing a pouch or something like that to put around my waist. Is that okay or will someone give me trouble because of it? I'll probably only carry my phone and some other necessities because they're easier to reach.
If you mean something like a Fanny Pack, then yes. If you're wearing it when boarding, it will probably be allowed, or not noticed.
However, it can technically count as a carry-on item, so, to be 100% sure, leave enough space in either other bag to squeeze it in just to pass the boarding Agent.
The pouch counts as a personal item which is something that most - but not all - airlines accept in addition to a carry-on. There is a possibly-incomplete list of airlines which do not accept personal items here. Unfortunately, the question was closed, so there was no other answers.
Check with your airline about acceptable personal items. From what I have seen, a waist pouch satisfies all airline requirements for personal items. Personal items can be much larger and often are things like laptop bags, camera bags, etc. Some airlines specify a size limit, some a weight limit and some both, so read the exact limits on the airline's website.
It seems you are already using a separate laptop bag in addition to your carry-on, so your personal item allowance is already taken. While I would not expect people to notice it and I nearly always see people with more than allowed onboard, it is better not to count on it. What you can do instead is just wear something with large pockets. Jackets and hoodies often have large pockets, some inside even for better security. If you are going somewhere warm, then use a vest. There are specialized ones for travel with many pockets. Since they are not available in Ecuador, I had mine custom-made but you can get ones from Scottevest. I finally tried one on in an airport and it seemed highly-practical and felt well-made.
If one of your bags is expandable, you can go with your pouch and place it into the expandable bag should someone complain. That lets you try what you want while having a backup plan.
I've worn a fanny pack on many, many a flight. The only time it's drawn the slightest attention was in the early days after 9/11 when they were doing additional screening at the gate. So long as it's sized such that you can wear it for the whole flight I doubt they would care as it's not taking up any luggage space, nor is it interfering with you in case of an evacuation (which is why they don't let you put something on your lap--evacuation certifications are performed without luggage, therefore they only apply if the exit path is free of luggage.)