[maddermax]

Yes, there is a limit. One unit, and one unit only can use a transport, as per the transport vehicle rules, P66 (under "transport capacity"). There are a couple of exceptions, like drop pods (which can carry two Combat Squads*) and storm ravens (which I hear tell can carry a squad and a dreadnought), but unless there is a specific exemption, then it's only one unit. That should answer your other questions as well. So, rather than taking 3 units of 5, you'll have to take one unit of 15 - and that's not a bad thing with Black Templars either

*note, Black Templars don't have Combat Squads, so their droppods are also one unit only.

Hope that helps

 

[maddermax]

What if i have the 8 man squad in the rhino, and i intend to have the HQ join the squad anyway, do i have to disembark the squad, move the HQ into coherency, and then embark the squad? Or can i just have the HQ enter the vehicle technically causing it to move into coherency with the unit its joining?

I assume you mean just an Independent Character as your HQ right?

Independent Characters can start the game joined to a squad if they choose (P48 of the BRB), and if they do so can start in a transport with the squad. Even though the transport might not be their dedicated transport, the rules on P67 for a dedicated transport make an exception for Independent characters that start attached to a unit, and specifically mention that an IC can start with a unit inside a dedicated transport (assuming there is enough room of course).

They can also board and disembark separately from the squad, joining it and leaving it as they do - as you say, it's just like moving into coherency. The character can just jump onto a transport carrying a squad and join that squad (assuming there's enough room), and a character joined to a squad in a transport can just jump off, leaving the squad behind if they choose to. Check P67, under "independent Characters and Transports" on P67.

Hope that helps