Look at the first part: "c_accaa" The letters alternate between 'c' and 'a'. After the initial 'c', the remaining pattern is a, c, c, a, a, which follows this order: c, a, c, c, a, a. We can assume the missing letter in the first gap should be 'b'.

Look at the second part: "aa_" We see two 'a's. The pattern suggests alternating 'a' and 'b'. Hence, the missing letter is 'b'.

Look at the third part: "bc_b" The letters alternate between 'b' and 'c'. The missing letter in the gap is 'b'.

The sequence with filled gaps is: "cbbccaaabcb"