Just saying 3 rides of 25km is kinda meaningless.
Regardless to whether you’re limited on time or not to improve then you need to be riding some of the time above the intensity that you race at. Therefore intervals are the way to go.
If its XC racing you’re aiming for then ride off-road intervals on a loop or on trails that you know. Anything from multiple reps of 4 minutes or so at pretty much max up to longer 20 minutes race pace reps. Include both hills and descents in these as well as “flat” sections.
Couple of sessions like that plus a longer (more than race time) ride is the fundamentals to a training plan. Easiest way is to hang with a good club and let them decide whats going on.
Training doesn’t need to be complicated it needs to be done consistently.