Can you fence seriously while working full time and only training two or three times a week?

Yes. Most adult fencers do exactly that, and adult classes run in the evenings for that reason. Two or three good sessions a week is enough to learn the sport properly, compete at local tournaments and, over a few years, earn a rating. What it will not do is let you catch teenagers who train daily with private lessons in open national events. Serious is about how you use your sessions and whether you keep showing up, not how many nights a week you are at the club.

Progress for a busy adult usually looks like this: a few months of footwork and fundamentals, a first local tournament within the first year that is humbling but fun, pool wins soon after, and eventually a medal at a small event or a first rating. Plateaus are normal and so are setbacks from work travel or a crunch month. You lose a little sharpness, not everything. Adults often out-think younger fencers even when they cannot out-move them, which is part of why the adult game rewards patience.

To get the most from limited time, arrive warmed up with one specific thing to work on, and treat bouts as practice for that thing rather than just trying to win. Ask to fence people slightly better than you. A private lesson every week or two speeds things up more than an extra group class, and a working adult can often afford one. Ten minutes of footwork at home a few times a week, some general conditioning and decent sleep fill in the gaps. Avoid jumping from two nights to five and burning out.

Adults have plenty of places to compete. Local open events take all levels, and rating-limited events such as Division III (D-rated and below) put you against people at your stage. Veteran events start at age 40, run in ten-year brackets, and go up to national championships and beyond. Check which age groups you qualify for with the eligibility finder, and confirm the current qualifying path for national events with your coach or USA Fencing. Plenty of adults never compete at all, and that counts as fencing seriously too.