If you think you're a Scrabble genius because you have all the two-letter words memorized, you're sorely wrong. To be a Scrabble genius, you have to know practically "every" word acceptable in Scrabble, so I'm guessing there's not too many Scrabble geniuses out there, because it's an impossible feat.
But the next step up from knowing all the two-letter words in knowing all the three-letter words. There are ten times as many compared to the two-letter words, but it's well worth the time to memorize them.
Here are a few key ones to remember from the OSPD (my favorite is CWM):
ABA, ABY, ADZ, AHI, ARF, AUK, AVO, AWA, AZO, BAA, BEL, BEY, BOS, CEP, CIS, COL, COX, COZ, CWM, DAK, DAL, DAW, DEX, DUI, ECU, EDH, EME, ETH, FEX, FIL, FIZ, FOU, GAM, GED, GHI, GNU, GOA, GUL, GYP, HAJ, HAO, HEP, HIC, HUN, HYP, ICH, ICK, IFF, IGG, ISM, JEE, JEU, JOE, JOW, JUN, JUS, KAB, KAE, KAF, KEA, KEF, KEP, KEX, KHI, KIR, KOI, KUE, LAC, LAV, LEK, LEU, LEV, LEZ, LUX, MEG, MHO, MIM, MIR, MOC, MYC, NEB, NIX, NOH, NTH, OBA, OCA, ODA, OES, OHO, OKE, OOT, ORC, OXO, PAH, PAX, PHT, PIU, POH, POX, PYA, PYX, QAT, QIS, QUA, RAI, RAX, REX, RHO, RIA, ROC, RYA, SAE, SAU, SEI, SHH, SOU, SRI, SUK, SUQ, SYN, TAJ, TAU, TEG, TEW, TOD, TWA, UDO, ULU, URB, UTE, UTS, VAU, VAW, VUG, VUM, WAE, WAW, WHA, WYN, XIS, YEH, YEW, YOK, ZAS, ZOA, ZZZ
For the full list of 3-letter words, visit the Scrabble 3 Letter Word List.