If you are two people then the Imam should have the ma'mum on his right (side-by-side) not behind him! So if a third one came (assuming you are talking about male of course or a congregation of the same gender)
and joins two who are praying, then the one who is praying behind the Imam should move back! (see linked fatwa)
Often this third person or second one who joins the prayer of the Imam indicates this by slightly touching the ma'mum on his shoulder, this is the same way it would be done if a row is complete and somebody want to join congregation pray but shouldn't stand alone in one row!
Of course it would be best to arrange behind the Imam and make him stand in the middle, if anybody comes later it's preferable that he joins from the right side of the row if possible.
...this is if the second person (who) joins the prayer (following the
Imam) during the qiyaam (standing). But if he comes during the
tashahhud or sujood, nobody should move forward or move back until
they stand up again. There is no dispute that this moving forward or
back should not occur until after the second person has said the
takbeer for starting the prayer, as they mentioned. (see linked fatwa)
You can find ahadith showing us how to pray congregation with only two persons, here from that sahihs Books (the Version from Sahih al-Bukahri), Sunan at-Tirmdihi and you'll find ahaidth showing how to deal if a third person came in Sunan abi Dawod and Muwatta' Malik.
You can also read this fatwa (where the hadith i only found in Sunan abi Dawod is referred to as in Sahih Muslim).