half-note triplets

If it's just the triplet that's the issue, see https://musescore.org/en/handbook/3/tuplets. However, I assume you are having a bigger issue considering the unusual syncopation you're describing. You can enter the bass as usual since it'll default to voice 2. I'm not a drummer, but I do know that typically ride and snare are both notated in voice 1, meaning that to get the rhythm you want in a single voice you'd need to use a custom tuplet. Your other option would be to write it in 12/8 instead of 4/4, though I don't know if that makes sense for what you're writing. Here's an example score I made to try and demonstrate what you could do, either in 4/4 or 12/8. The first and third measures are the rhythm you described exactly how you described it, using 3 voices. The second and fourth show it with the ride and cymbal combined into one voice, using a 24/16 tuplet in the 4/4 measure. No matter how you notate it, it's gonna look messy, but it is at least possible.