Stress will cause the fish immune system to weaken, thus more susceptable to whitespots... Acclimatize them the right way will reduce the stress the fish go thru... 3mths is still a very new tank, it takes about a year for marine tank to mature and stablize. Keep ur water parameters in check, acclimatize the right way, do not over stock, add fish wisely, its a process every reefer have to go thru... Take ur time and be patient... It will pay off
As for white spots, its probly in your tank already, and its there to stay unless u do something about it. Since u have not much fish now, stop adding them, let it go fishless, and i mean FISHLESS, for at least 3 weeks... The parasite will die without having any thing to host. Then u can slowly add fishes... If not its only gonna to get worse, the more fish u add, the more stress, the more ich... Thats just my opinion, dun be disheartened, scour the forums and learn as much as u can for the time being. Good luck