I currently have a math, an English, and a computer science teacher as my recommenders. Some of my colleges have slots for 2 teacher recommendations and 1 other recommendation. Could I assign one of my teachers to be that other recommender? Does the subject or the college matter for this?
While systemically you can, the college will prefer that the other recommender not be a teacher. 3 LoRs telling them how brilliant you are in the classroom is overkill
If you use the third LOR it should be for someone who knows you very well outside of the classroom and can speak to a different aspect of your personality/qualifications etc. For example people may ask an employer, coach, music teacher or someone along those lines.
Do you think it would it harm my application if I put a teacher as an other recommender? Especially if I get another outside recommender if a college has 2+ available slots for other recommenders?
Read the instructions for what each college wants carefully. Typically the max should be 2 teacher LORs, 1 guidance counselor LOR, and (if appropriate/useful) 1 supplemental LOR.
Do not use a teacher as a third recommendation unless that teacher knows you well through an extracurricular activity, such as being a debate coach.
But ask your guidance counselor if they accept “in house” recommendations, where your teacher submits a recommendation to the guidance counselor, and the counselor summarizes that recommendation in the counselor letter.