Recipe(tried): freezing tomatoes, peppers, and onions for salsa

You can freeze them, but the texture will be softer since the cell walls break down during freezing. Measure them out in the amounts before freezing them. Many people do this.
As a side note, dicing up all the peppers ahead of time and freezing them works great ! I do the same thing, just premeasure and freeze in ziploc bags, labeling them to the type and amount.
You can also dice onions and freeze ahead. They are only good in cooked foods, not to be used as raw.