きのうわたしのうちにおきゃくさんがさんにんきました。
This is perfectly correct and natural (except that you used no kanji).
きのうおきゃくさんさんにんはわたしのうちにきました。
So you made three changes: 1) used は instead of が, 2) moved さんにん, and 3) moved わたしのうちに. Among this, 3) is perfectly okay, but you usually should not do 2), and you should never do 1).
- You have to use が instead of は because "guests" has not been introduced in the discourse (See: What's the difference between wa (は) and ga (が)?).
- さんにん is basically adverbial (i.e., modifies a following verb), so it is usually placed somewhere between が and きました. Instead, you could have said さんにんのおきゃくさん to say "three guests" like in English, although it might sound a little stiff and unnatural in a casual conversation (See: How to list numbers of things)
The following sentences are valid alternative ways to say the first one:
- 昨日お客さんが3人私のうちに来ました。
(きのうおきゃくさんがさんにんわたしのうちにきました。)
- 私のうちに昨日3人のお客さんが来ました。
(わたしのうちにきのうさんにんのおきゃくさんがきました。)