It's a legit concern. In the last game against the Jazz, Utah was +18 with that lineup. It was also the lineup they used in the fourth quarter to make their comeback. Since the Spurs only lost by six points, it was likely the difference in the game.
It's possible Pop could try something like Diaw at small forward to combat it if it becomes too much of a problem but I think that Leonard and Jackson are big enough to hang. The key will be to have Duncan on the court so he can clean up the boards. Without Duncan last game, the Jazz just lived on the offensive glass.
I'm not sure if Leonard will be able to defend one of those three but I'm pretty sure Jackson is up for the challenge. He's tall, strong and compe ive enough that he'll be able to survive.
The real key will be to make the Jazz pay on the offensive end. While Millsap is mobile for a power forward, he's slow for a small forward. Spread the court, utilize a lot of movement and make those bigs chase players off the three-point line. The Jazz aren't very good at defending the three-point line as it is. With those three on the court at the same time, the Spurs should get plenty of wide open looks from deep.