It not so much a loose standard as it is a 3” tube was an option that was available from Dexter and maybe others on the 3500 lb. axle…
as an option it wouldn’t be found on many trailers with 3500 lb. axles, I doubt the 3” option was widely used by the RV industry especially…
the RV industry as a hole seldom upsizes anything because its better, but more likely out of some minimal need…
typical, tube sizes can be found in the Dexter 600 to 8000 lb. catalogue on page 113, Or the Eastern Marine trailer parts superstore, under axles & axle hardware…