I'm not an expert but if the Colts are +3 +108 and not -108 you have to pound it. Each $2 risked will win $100 or if Indy covers or wins outright it's a free shot. If the Colts are -108, it's a borderline call either 10-1 or 12-1(approx) shot that KC wins by 3. If you saw the a prop bet written: KC will win the game by EXACTLY three points +1000 or +1200 would you bet it? I've seen charts with point differential and I think 3 hit 6-7% of the time, but keep in mind if Indy wins by 3 you lose. However, I think it would be a higher percentage in game in which the spread was 3. In the long run it's probably not profitable, but you can take a shot...I don't think it's a terrible bet.
Same deal for the other game, 20-1 shot that Philly wins by 4 or 5. It's probably very close if that;s a long term winning margin, but not a terrible bet if you want to take a shot. I'd throw 200 on each side and risk $20 to win $400. Good luck.