First month,Last Month & Security Deposit.

Can Landlordask for First,last month and security deposit when a tenant at will rent or this apply to leasing only? or both??
I as landlord only ask For First Month and security deposit.

The last month rent must be placed in an escrow account according to massachusset law and
I Would like to know if the last month rent can be used by tenant ? or is it used as another deposit and then return it when tenant vacate apt?

What would happens if tenant want use the last month and then stay and the last month is not longer in reserve?

Your advise is well appreciated. Please advise me.