Welcome to the forum, Mikey!
If all three of the assignments you are cancelling are going thru one MSC, then contact the scheduler ASAP and do your best to reschedule. This is not something you want to have to deal with at any point, but it is especially worrisome when you are first starting out and trying to prove yourself as a reliable shopper.
What happens with one MSC does not affect your rating/status with other MSCs. At least officially. Schedulers talk, just as we do. So keep cancellations to a minimum, and only in the case of real emergencies.
Good luck!
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"Between stimulus and response, there is a space.
In that space is our power to choose our response.
In our response lies our growth and our freedom."
~Viktor Frankl