Picked up cruiser that has a pair of coaxials on right side and a single coaxial on the left, and a sub. There is no place to add another coaxial on the left for an even balance. Notta, nothing, zilch. On the left side, the speaker is located in rear of the cockpit behind passenger bench seating. The right side has 1 located right next to driver, and the other also in the rear of cockpit behind drivers bench seating.
How would I connect the 3 speakers to a 4 channel amp without creating a mismatch of ohms/levels?

If your just connecting the 3 coaxials to one amp then just hook one speaker to one channel etc and leave one channel not used....each channel has its own ohm requirements...but that doesn't mean it needs anything hooked to it to work properly...so you can leave one channel not hooked up and be fine with 3 speakers/channels...you just wont get front/rear or left/right balancing depending on how the rca's are feeding the amp.
If you are concerned, you could bridge two channels together and feed the single coaxial speaker with some extra juice but you would have to check with the amp mfg to see what kind of ohm loads bridging can handle.
Ive run a 4chan amp with only two speakers and the two other channels not hooked up/used and it had no problems...even took the amp out years later for another project and hooked the two other channels up and still worked great till i sold that truck.

What Wally said will be work if you can't add another speaker.

Can you add another speaker in one of theses two places??

Thanks. Did not know I could leave 1 channel unused.
No, I can not add in those locations. The forward spot is the doorway/wall to the cabin. The rear location could work, but would be really tight. I'll just run the 3 and call it good.

that looks like a really nice boat !
how about putting the second port side speaker in / partially in depending on depth or on the arch ?
whats that panel next to the grill, i'm sure there's a spot

^^^^^^^^^^ Arch.

Naw. My stereo days are over. If I put a speaker in the arch, then I’d have to do both sides and be right back to where I am now with an odd amount. . Not even sure if I’ll hook the sub up as don’t want to mess with another amp. Just want the existing speakers to work.
Thanks

Bridge the 4 channels to 2 and run those to left and right and see if you like