Combine the zest and juice of 2 lemons, oregano, red pepper flakes and olive oil in a dish. Paste the garlic by mashing it with coarse salt then add 3/4 of it to the marinade, reserving some for dipping sauce. Add chicken tenders to the marinade and season with salt and pepper, turn in marinade to coat, let stand 10 minutes. Thread tenders on damp skewers and cook 7 to 8 minutes, turning once, until firm and juices run clear.
While skewers cook, combine yogurt with juice of remaining lemon, remaining clove of garlic, grated cucumber, cumin and salt, to taste. Divide the dip among 4 ramekins.
Serve 5 skewers per person with dipping sauce alongside.