1) You can add an information segment to link the 2 PNRs. Note however this will not protect your connection. If you misconnect you could still potentially be on your own. The only way to protect a connection is to book all on one ticket. Almost as good is fully flexible tickets.

2) Yes. Check your BPs carefully.

3) 3 hours in LAX should be enough time to connect, even with a change in terminals. Whether you want to take earlier flight (longer connection) depends on how much risk you're willing to take.