Are there any specific requirements (physical and otherwise) for 03** MOS' ?

When you enter Boot Camp and beyond, you need to train hard both physically and mentally. Training may be hard, but combat is harder. Earning the Eagle, Globe, and Anchor along with the title of Marine is worth the effort.

Can you pass the IST, PFT and CFT? If not you need to focus on those areas for starters. You will also need to be able to go on a hump (hike) with a pack and also be able to carry your Primary weapon system which would be either a 60mm or 81mm Mortar if you are actually signed up for Mortars. But if you have an 03XX contract it means you will be Infantry but what specific MOS depends on how things pan out at SOI.

Well....if you're signing up to work with Mortars....you're gonna need to be physically fit to hump around Mortar equipment and lugging the rounds from storage to tube for fire.