the thread says first three mods, so i'm guessing that means mods to a stock vehicle and not gear....
as mentioned already, TIRES. even when i wasnt going offroad much, i would buy a new vehicle and immediately put better tires on it. my current vehicle is a tundra and i wanted a bit more clearance, so i went with a taller tire (needed a level kit installed for tires not to rub) and a better tread.
its like hiking, running, etc....you need the right shoes
next for me was power. i added a second battery and chose to go with a solar panel on top of the camper shell to charge it. either that or install a second battery that hooks into the alternator. the extra power is great for lights at night, using a 12v pump for water and being able to charge phones and batteries for flashlights and stuff. we sometimes sleep in the camper shell, so that extra battery to run a 12v fan is really nice and the ability to run a 12v fridge.
third would be storage. pretty much any stock vehicle will need modified storage. some add pouches to the backs of the seats or boxes with drawers that can be taken out when not camping, etc. people with pick ups will have a decked system or rack mount in the bed, etc. basically use whatever is a good way to store and secure gear for your specific vehicle.
i have a stock tundra and went with a taller tire, camper shell with deck system and 2nd battery with 50w solar on top of shell. these few mods have been working great for a couple years and has been more than enough to handle everything we have been doing
now...if youre married and/or have kids, then the mods are pretty much anything that keeps them from fussing and will change with every outing....good luck