Should i start taking immune boosters before testing positive for covid 19 (ie vitamin c, vitamin d & zinc)? i.e can they be taken beforehand or should it only be taken if tested positive for the virus?

Immune boosters ?'s : Vitamins C , D and zinc will not prevent co-vid infections, and there is no definitive clinical data that such additives will diminish a positive exposure /test for covid-19.

No harm: Little benefit. There is mild evidence that low vitamin D levels may predispose to Covid-19. You may take these but do not expect these medications to promote your health or immune system. Have a healthy diet, exercise 30 minutes/day, drink plenty of water daily so your urine is mostly colorless, have safe sex, no tobacco alcohol weed or street drugs. For nutrition: http://www.choosemyplate.gov/