It would be helpful to see some other angles. The trunk that goes to the left is kind of straight and uninteresting. From what I can see it looks like all the trunks go out at a wide angle from each other, so even if you cut off one trunk, the two remaining wouldn't look very good. Having said that, the tree isn't all that bad.
With what little can be seen my suggestion is to leave all three trunks for now, let it grow out some more and keep evaluating. Pyracanthas are one of the toughest things growing, so it you decide to drastically prune it will come back with a vengeance. I love a clump and multiple trunk styled bonsai, but they are almost as difficult to do well and a good bunjin
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Carmen