Surfing Waves

hello im a beginner surfer I love surfing and using friends board to go surfing I just bought one on craigslist for $30 first time I went out with it the right side fin broke off its a tri fin board
should I just cut off the one on the other side or try to find another fin im thinking of just cutting the one off the fins are sopposed to be like this I I I but thiere like this I I so the middle and left fin are fine...shouls I chop the left one of and path it with fiberglass repair kit I have? please reply quickly!!!!!!!!!!!!!!!!

Since it stands out that you know very little about surf boards, ( and that's OK at this stage) take the board down to your local surf shop,get a quote, having a new fin put in will be better than cutting off the other fin.
Normally thruster set ups ( that's three fins) have three equal but smaller fins than a single fin, so the tail is going to continually break out , equally as disastrous is you putting on a new fin but having it at the wrong angle etc, if the board was any good it will become a dog if the fin is wrong.
But if you want to start an understanding and appreciation of surfboard making you could have a go just make sure you match the positioning of the fin and it's angle in ( toe in ) and it's angle of lean, it could be the start of something for you!
Post pictures of your board, your weight and height, then we can advise you if the repair is worth it, a new fin will be at least $30.