All-in Rule Question

New here and I see plenty of things on this site that will keep me coming back.

I run a weekly no-limit tournament at the local VFW for the vets. I had the following event take place and it seemed to bring a lot of distaste between the players. I would like some feedback and possible someone to guide me to were I can find an official ruling.

Blinds are at $100 / $200

SB and BB are posted, player 3 goes all-in pre-flop with $300, covering the BB but not fulfilling a legal raise.

Is the bet now $300 or do the remaining players have to pay $400 to see the flop?

I believe they would only need to call the 300 total to head to the flop. I am not 100% on this though.

You would have to call the stacks of the all in given that three people have gone all in. Either you would be all in or be covering their stack on the call.

The remaining players have to pay at least 1bb in the first round preflop. So they can call 300, 300 is a legal raise from the all in player.
Their min raise would still be 400

If the players have good cards they can raise or call the raise. Probably the player with the 300$ had no other chance and he went all in trying to steel the blinds!

They would only have to call the $300.00 as that is a legal raise for the all in player.