Does there need to be a common theme for STEM triple major?

His college has a small amount of graduate students so undergraduates are relied upon for research projects, to the extreme that the policy is “ Check out the research summaries outside of room 201 and talk to faculty.” He is planning on being involved in physics research on campus next fall and to look into the REU program for the summer after his sophomore year. Did your child start the REU program after freshman year or sophomore year? He thought he was more likely to be accepted after next year once he declares his major(s) and has another year of college experience.