First realize that the $a,b,c$ in the sentence are constant symbols, while the $a $ and $b$ in the domain are objects, and these are different. That is, an interpretation needs to map each of the constants $a,b,c$ to either one of the objects $a$ and $b$ ...which is a little confusing... But can of course be done. And, once you have decided on such an interpretation(say, constant $a$ maps to object $b$, while constants $b$ and $c$ both map to object $a$), then the truth-functional expansion of your sentence with regard to this interpretation becomes:
$(P(a) \lor \
eg P(a)) \land (P(b) \lor \
eg P(b)) \land ((P(b) \land P(a)) \land P(a))$
In here, I use a 'new' constant symbol $a$ to refer to object $a$, and 'new' constant symbol $b$ to refer to object $b$, just as you would in any truth-functional expansion.