IVY and top academic D3 do not offer anything but slots for D3, or likely letters for IVY. If the coach at the D3 did not say he is giving you a slot, but only support, well you had better have a back up plan. It is up to admissions, you are just another applicant who will play lacrosse once you get admitted.
Ivy if you do not get a likely letter, you are also in the regular admittance pool of applicants.
Keep your options open, keep talking to others.
http://www.tier1athletics.org/category/division-3-recruiting/