trio friend group

i have a friend group of 4 at school, but when we hang out outside of school one of them has a pretty busy schedule so most of the time its just us 3. i feel pretty left out as the other two talk to each other a lot and not as much with me. its to the point where i only want to hang out with them if its all 4 of us together, but idk what to say bc the other 2 have invited me to hang out with them. i feel like telling them that i would rather hang out when all of us are together, but i don’t want to sound like i hate hanging out with them.