With fresh foods, it isn't just a matter of what kinds and how often, but the most important aspect is how to safely introduce them. Whenever a new-to-them food is offered, a rabbit's gut may need time to develop the enzymes to digest this new food. Each rabbit will have different tolerances to various greens as well as different likes/dislikes.
So the key is to introduce just one new food at a time over a period of several days. The process is described in the link below. (Your rabbits would be considered 'adult' at their age.)
Bear in mind that fruits (and even carrot) are considered treats because of their sugar content. They are not considered part of the "greens/veggies" that should be part of a rabbit's daily diet. The fresh part of a rabbit's diet should comprise of mostly leafy greens so think more in terms of "greens" than "veggies."
Once they've been introduced properly (per link below) to a variety of greens over a period of time, you'll have a list of what greens your rabbits tolerate and enjoy. Once that point has been reached, they can have about 2-4 cups daily. Those can be fed however you like -- throughout the day, divided into a couple feedings per day or just once per day. They can even be used as a reward for any training you may want to do.
https://rabbitsindoors.weebly.com/greens--veggies.html