Hi Travel ,part of the functioning of thrusters was based on three equal sized fins sharing the turning qualities, so I'd replace the missing one as without the dimensions of your board you can change the performance dramatically and not necessarily for the better. Better to try different sets
If you have a centre fin box as opposed to the FCS plugs then there could be allowance for adjusting the position of the centre fin and this would allow you to play with different sizes. It is an interesting interplay between rail, tail width and fin sizes and you might want to buy several sets of different templates to try.
If you can, some surf shops have different templates to try, can be a great learning experience, if you surf with mates try swapping amongst yourselves.
Take some time experimenting you are likely to find out much more about your surfing that way.
I've taken up troll hunting just for fun, instead of a rifle I'll just use a pun! 冲浪爷爷