If the visa is for 27 months but you only need 24 months to be physically there, is the 3 months a grace period of when you would like to enter or do you have to enter the UK the day it is issued?
No, the 3 months is extra on top of the 3 months and 28 days you have once the visa becomes valid... so technically you could give yourself a total of 7 months between your application date and entering the UK.
Technically, you can enter the UK on your visa at any time during the 27 months... however, if you leave it for longer than 3 months and 28 days after the valid from date, you won't be able to apply for ILR in 2 years (because your visa will expire before you qualify for it and you will have to pay for another spousal/FLR(M) visa).
Let's say I apply for a spousal visa with a post date of July. If the visa is granted, is the first three months a grace period, that's 3 of the 27 months visa?
Say you apply and post-date it July. You will then have until November to enter the UK on your visa and still be able to qualify for ILR before it expires.
Obviously, though, the earlier you enter the UK, the earlier you can qualify for ILR

.