It becomes clear from Bereshit 18:2 that Avraham is looking around from the entrance of the tent and sees three men appearing, cause it reads: והנה שלשה אנשים נצבים עליו.
But what did Avraham see the second time the word וירא appears? It only says ‘and he saw’, it doesn’t say ‘when he saw them’ (והוא ראה אותם) or ‘as he saw them’ (כפי שהוא ראה אותם). And although it becomes clear he ran to great/call/meet them, it doesn’t seems to be connected to the fact that ‘he saw’, for he already saw them in the first part of the passuk. So what did he see the second time he looked that made him run towards them instead of running towards them right away?