the function IF uses

if( test, true, false)
so IF
D17=AB6
OR
D17=AB7
is true then you get
"Target Change: "
otherwise its false and you get
"Current Target: "

Now you want to bring in a third option
OR(D17=AB6,D17=AB7,D17=AB8,)

D17=AB6
OR
D17=AB7
OR
D17=AB8
any are true - then you get
"Target Change:

So what are the next set of rules to work out if
"Current Target: "
OR
Suggested Target: "
should be used

does that help

you can use nested IFs

so
IF( TEST1, TRUE , IF(TEST2, TRUE, FALSE))

so if the result of test1 is false - you can then do another tests
or use true

IF( TEST1, IF(TEST2, TRUE, FALSE), FALSE))