Kendall Jenner and Blake Griffin are playing it cool in the midst of an L.A. heatwave ... and also with what sure as hell looks like their new relationship.

For the third time in a week, Kendall and the hoops superstar hung out together -- this time they were beating the heat on the beach in Malibu. She was laughing and looked smitten, but still no PDA ... on camera, anyway.

Just like their other dates ... Blake and Kendall were with a group of friends. Neither's confirming they're a couple, but the "just hanging with our pals" cover only works for so long.

Something's cooking ... besides Blake's balls. Jeans on the beach in 90 degree temps, playa??!!