it seems you are talking about 2 different things here, time signature vs. groove - first you want to know if you can do 3/4 without H. well sure, keep groove set to 6/6 and just use 3 phrases for every 4 bars instead of 4 (12 16th notes x 4 bars = 48 lines; beat 1 of bar 2 starts on line 0C of the first phrase, etc). however this won't make it as easy to remember which bar of the song you are sequencing, and if you're not careful it might cause your chains to come out of sync. just depends on how you want to program it. visually it's easier to see where the end of each bar is if you use H at 0C instead.
changing the groove changes how many ticks each line in each phrase plays for. it won't change what time signature you are writing in, it will only change the feel of the time signature. you could still write a song in 4/4 with a groove set to 7/5, 8/5, 8/4, etc. around 66% swing it will sound more like 12/8 than 4/4 (or 9/8 than 3/4).
either way, say if you are writing in 3/4 with a 6/6 groove but you want to write eighth note triplets, you'll either A. need to use D commands to get your notes to play on the right subdivisons (6 ticks x 4 lines = 24 ticks, 24/3 is 8, so to get 8 ticks per note you would need a D02 on the second note and a D04 on the third) or B. use G to switch to a different groove set to 8/8/8 and then switch back.