Successive Triplets

I'm still new at Musescore, so I might be missing something, but it seems way too hard to do this.
I'm in 4/4 time. I want my next measure to hold 12 notes of equal duration, say AAABBBCCCDDD. I go into Note Entry mode, set the duration to quarter notes, press Ctrl-3, and I see 3 eight-note rests. All is good. I type A 3 times. Then I type a B for the next note, but the triplet mode is gone. I just get non-tripleted eight note. So, I type ctrl-3 again, but instead of a quarter note length triplet, I get an eight note length triplet. I tried many combinations of highlighting different rests, doubling and halving durations, but nothing works unless I leave Note Entry mode, and declare a new triplet, which gets me back to note entry mode. So I need to leave Note entry first for the 3 B's, then the 3 C's, and again for the 3 D's. Am I missing something?