Cab Use Just One Pole of MCCBs

I disagree. It may be ok to do this with a relay. NOT a 100 amp circuit breaker

Option two would be to use a three phase 100 amp unit and parallel the three lines to work as a single 300 amp unit.

In most national electrical code books paralleling three, and in some cases as many as six, medium sized units to work as a single larger amperage unit is allowed.

Yes, but...

At least in my code (the Canadian one), parallel single phase breakers, operating at 750 Volts or less, must be assembled as such by the OEM. They cannot be field assembled and used.

A next solution could be to turn the 3 phase 300 Amps into single ones. Their levers are mostly connected together with a removable device: be it a pin that connects the 3, or a bridge that fits on top of these.

Leaving this connection in place will make the 300 Amps not correct, since only one action in 1 breaker needs to overcome 3 loads. In a bad scenario this one connected part could catch fire before disconnecting I guess.

A handy electrician can fix this. Just don't know if it is OK with the code. Please advise.

Using any 3 pole cb for single phase will work. A 3 phase breaker will not always have a 3 phase fault. Any one of the phases are manufactured to trip all 3 at the rating of that circuit breaker. Whether it qualifies as a single phase disconnecting means is another matter.

TCM - If I did that, ( 3/100 ) I would always worry that sooner or later ( probably sooner, just as I am putting my boat in the water on my first day of vacation ) the weakest of the 100A poles would trip out, and I would be called on the carpet for undersizing the MCB.

If I installed a 3/300 breaker I would series-wire the poles, to occupy the lugs.

I just go by what I have seen done in commercial panel applications. So far tying all three circuits of a three phase breaker together to make a larger single breaker is a very common and acceptable thing to do especially in special application circuits.

Countless home electrical breaker panels have this sort of setup for the primary breaker. Many use four breakers set up as two parallel smaller breakers tied together on each leg ganged together to make a single 150 - 200 map main unit and I have never heard of one false tripping due to just one of the two on a single side getting weak.

To be honest at those current capacities they are rarely ever loaded down hard enough or long enough to trip anyway.

Whereas if you are using three 300 amp breakers all in series you have 6 connections plus three sets of breaker contacts all having to handle up to 300 amps each without getting a hot spot or the combined resistance not creating a overheating issue.

Using three 100 amp circuits in parallel doesn't have a single connection or contact set carrying over 100 amps which means far less peak heat being generated in the unit.

Up front cost and connections wise using a three phase 100 amp breaker as a 300 amp single phase unit cost less and makes for far less wiring work plus distributes the load thee ways instead of no ways.

...will it work? yes.

but depending on your code, it may not be allowed. depending on whether your inspector knows the code that well. .....and as for paralleling, that may be against code also, depending on wire size you intend to parallel.

Common practice go right ahead. Just label it.