Is 'Employment Prohibited' common on Workcation visas in Thailand?

You can change your itinerary once after the visa issued as long as you plan to enter Thailand is within the visa validity as Tod explained.. No need to inform about changing your flights

A 3 month visa what is the visa name ?

What Visa ?

Your visa when issued is valid for 90 days, you can enter Thailand on any one of those 90 days.

That visa is valid for 3 months from the date it's issued meaning as long as you enter the country before the visa itself expires you will be stamped in for the number of days that visa allows when you arrive,

even if you were to show up on the last day the visa was valid for you would still get stamped in for the correct number of days on entry