Regarding fish oil - there's a usually massive difference between a good brand of fish oil and something you might pick up at Whale*Mart. For example, comparing a bottle of wal*mart fish oil to my NOW brand fish oil, 1 cap of NOW has the same amount of EPA/DHA as 3 wal*mart caps. Find a good brand that has a lot of the good stuff, it usually costs about the same anyway.
Ok so being a long time supplement whore i have some suggestions..
Whey/egg/Casein blend: Optimum Nutrition Pro Complex (rich milk chocolate is always safe) best protien i have ever used.. the new Hydrobuilder is good but if you want to gain mass the Pro complex has more protien and calorie content.. so skip the mass gainer and just use the pro complex in my opinion.
Multivitamin: Gaspari Nutrition Anavite all the way... best multi i have personally ever used with full doses of Beta-Alanine and L-Carnitine L-Tartrate.
Omega-3's if you like to order offline then i would say Nordic Naturals Ultimate Omega, 180 Softgels from BB.com if you goto vitamin shoppe they carry Udo's i personally like the liquid but some people hate the taste.. but they carry gel tabs also. If i had to pick only 3 basic supplements those would be the big 3: O.N. Pro complex, G.N Anavite, and Udo's Choice Udo's Oil 3 6 9 Blend.