The Red Sox got help from star David Ortiz, who went three for four with an RBI double and is now hitting .733 in the Fall Classic.
I have no knowledge about baseball statistics so please help me understand this sentence.
"Three for four" refers to the batter (David Ortiz) reaching base safely by means of a basehit (single, double, triple or homerun) three out of the 4 times he has batted in the game they are referring to.
"an RBI double" refers to Ortiz getting a hit (a double) and reaching second base, which caused another runner to score (an RBI - run batted in)
"is now hitting .733" - refers to his batting average in the World Series games only.
"the Fall Classic" refers to the World Series.