He's not dense, a dense character doesn't notice these things. The MC here is in that sweet spot between too stupid to get the hint, and too smart to follow through. His common sense is telling him that there is no way, the three guys that bullied him as a kid, could be three beautiful girls that are into him now, and he definitely didn't just go on a hiking date with the real Fumi-tan.He was so close to putting 2 and 2 together. Alas, the harem romcom protag denseness struck again