They have a 4-3-3, and they've been using it. Ribery and Robben ageing on the front 3, and Xabi in the middle 3 doesn't help... Robben might have a season or two in him yet, but Bayern need to find replacements for those 3 primarily. Sanchez and another world class AM/wide striker will work, but I'm sceptical of Kimmich replacing Xabi (or rather, Kimmich being a world class midfielder that Bayern need him to be if they want to run vs Real, Barca, Juve and PSG). FWIW, Kimmich said he'd rather be Xabi's successor than Lahm's at RB. The system is there, it's just that some players need to be replaced next season.