With the score tied 4-4, Ausmus's only hit was his second homer this season leading off the inning.
Later with two outs, Erstad hit his third homer. He had two of Houston's eight hits.
Hunter Pence hit a two-run, fourth-inning homer for the Astros, winners twice in the four-game series.
The winning pitcher was LaTroy Hawkins (2-0).
Jose Valverde earned his 33rd save with a 1-2-3 10th.
New York's Carlos Beltran homered twice and knocked in three runs. He hit a two-run shot in the first and a solo homer in the fifth.
Fernando Tatis had one RBI for the Mets, who had just seven hits and saw their lead over Philadelphia atop the National League East dwindled to one game, pending the outcome of Sunday night's Phillies-Los Angeles Dodgers contest.
The losing pitcher was Pedro Feliciano (2-4).