Granted every company is different and supervisors / managers have different philosophies on these things. But I can tell you there are companies / managers who would be absolutely livid if their brand new hire came in and said "you don't mind if I take a few days off over the next 3 months, do you?"
Speaking as someone who has been in management positions and done a fair amount of hiring, it really should have come up during the interview (if it didn't.)
At this point I would say you have two choices:
1. Call the person you interviewed with and inform them of the trips immediately. "I forgot to mention it when we spoke previously but..."
2. The other option is to ride it out until you are hired and then essentially let the employer make the decision for you. The last thing you can do is go in on Day One and start insisting that you be allowed to take the trips. Most companies have probationary periods during which you can be let go for any reason. Most do not allow employees to take time off (even unpaid) immediately. You may have coworkers with conflicting days off which prevent them from even give you the time off.
Well, I guess there is a third option--cancel all of the trips.
Again to put on my manager hat...one pre-scheduled trip for a new hire is workable. Three trips is pushing it.
There are vast differences between different companies and industries. If this is a company where schedules are flexible and people are workers are permitted to come-and-go to a degree as long as the work gets done, it may go over easier. But if it's a company with more rigid scheduling, high expectations, or even seasonal peak times (Christmas), you really risk getting off on the wrong foot with management and coworkers.
Good luck.