This Language says, that last 3rd digit is a always and all others can be either a or b.
So, Minimized DFA has $2^{3}$ states = $8$ states
Property :- When $nth$ character from the last is always either 0 or 1(I your case always a or b), then Minimal DFA has $2^{n}$ states.
You can try to design a DFA.
A lookalike https://gateoverflow.in/65645/mininmal-dfa