This is a fluke, as normally #mIRC.1.network.log would be merged with #mIRC.network.log when you leave the channel (whether it be by part, kick, or quit).
The fact that you have multiple .number files means that the .1 log wasn't deleted.
If you want, you can look through the #mIRC log and see if the .3 and .2 logs were merged, just not deleted when they were .1 logs, and if they were, you can safely delete them.
Alternatively, you can merge the files yourself (risking duplication) and then delete them, or just delete them (risking loss of logged informatoin).
