$150 won't buy even one mic that will truly do a cello trio justice (you need at least a matched pair or a stereo mic), and there is a lot more to recording instruments than throwing a mic or three in front of the players with their instruments. You can get some decent studio time/hours at an indie studio (like mine) for $150. If you are thoroughly rehearsed you should be able to record and mix quite a bit of material in a long day.
The "DIY" method:
If you have access to a nice music room at school, or even the stage, that would be a good place to record. Or a "wealthy" friend with a large study or living room or even a traditional music room.
You may want to consider renting. You can rent a nice stereo mic from Trew Audio ($25 - $40/day) and plug it into a Sound Devices 722 ($75/day) and get a quality location recording. Spend some time auditioning levels and the mic position, then just start tracking. It's really "old school" but a lot of professional orchestral and choral performances are still recorded this way, and they sound great.
The reason I recommend this path rather than purchasing is that cellos are subtle instruments and cheap mics will not do them justice, just as the cheap preamps of most budget recorders and computer interfaces will impart a lot of hiss. You still will have to mix and master before you put out the CD, and the cheaper the mics and pres you use the more you will have to fix before you mix and master.