If they're the technical people and the ones building your MVP, then the work they're doing is now; they are building your product and probably your company's only value at this stage. Why would they wait 3 months for an equity agreement on that work?
Savvy entrepreneurs also generally dislike waiting for equity grants because they realize the tax implications of delaying the grant date - if you make an 83(b) election on equity (which you usually should, because the alternative can be far worse), you pay tax on the initial value of the grant. If that's high, you're going to get killed in taxes the year that you join. Ironically, it's their work which is raising the valuation of the company through completion of the MVP, so they're basically creating a tax liability for themselves if they delay the grant.
If you want to compensate them fairly, figure out the equity now and put a 6-month cliff into the agreement. (A more ordinary cliff is one year, but as a techie I wouldn't sign a one year cliff in the building stage myself - too much initial work would fall on me to justify a delayed reward).
You can always put in a provision to review and adjust later, or base the amount granted on milestones rather than time.