When all else fails look at the umpire manual. Page 79 of the 2019 USA Softball manual
USA.1803 Umpire Supplement FINAL2-HIREZ
And for the tag up responsibilities (leaving early on a caught fly ball) when no umpire chases, U3 takes the PU responsibilities and U1 has the base umpire responsibilities in 2 umpire mechanics.
Leaving early on the pitch is in the umpire manual also.