There’s loads of seats available so should be able to buy 3 together. However if not if there is a split it is shown on the plan but not all blocks have a split (compare 132 and 133 to see what I mean).
Log into tickets.brightonandhovealbion.com then click on your name in top right corner. Then scroll down that page it takes you to (which is account details) and there is your friends and family. You need their fan number and postcode (I believe) to add anyone that isn't already listed.