Walgreens.com has Buy two get third free on different Crest toothpaste and other oral care products. On their coupons page, there is a $5 off coupon (restrictions apply). Combine them and select store pickup to save on shipping.
Whitening Toothpaste with a Touch of Scope4.6OZ - 3.79 x 2 - 5.00 = $2.58 for 3 toothpastes. Plus tax as applicable in your zip code.