SQL Server 2012 :: Conditional Join Between Two Tables

I have two tables tabA (cola1, cola2, cola3) and tabB(colb1, colb2, colb3, colb4) which I need to join on all 3 columns of table A.

Of the 3 columns in tabA, few can be NULL, in that case I want to check the joining condition for the rest of the columns, so its conditional joining. Let me rephrase what I am trying to acheive - I need to check if the columns in joining condition is NULL in my 1st table (tabA), If so I need to check the joining condition for the rest of the two columns, if 2nd column is again NULL, I need to check the joining condition on the third column.

What I am trying to do is as below. Its working, but is very slow when one of the tables is huge. Can I optimize it or rewrite in a better way ?

--- First Create two tables
Create table tabA
(cola1 nvarchar(100), cola2 nvarchar(100), cola3 nvarchar(100))
Insert into tabA values (NULL,'A1','A2')
Select * from tabA
create table tabB

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