Your MSI Motherboard is compatible and will support the Ryzen 9 3900X: Support For B450 TOMAHAWK | Motherboard - The world leader in motherboard design | MSI Global
But before you install your new Ryzen 3900x you first MUST UPDATE THE BIOS/UEFI and CHIPSET to the latest versions.
You must have BIOS version 7CO2v18.zip or higher installed BEFORE you plug in your 3900X otherwise it won't boot up. The latest BIOS version is 7CO2v1D:Support For B450 TOMAHAWK | Motherboard - The world leader in motherboard design | MSI Global
Also check to see if the RAM MEMORY you have now is listed as being compatible with the new Ryzen 9 3900X Processor QVL List for RAM MEMORY for the Ryzen 3xxx Processors: Support For B450 TOMAHAWK | Motherboard - The world leader in motherboard design | MSI Global
Note: If your RAM MEMORY is not listed, see if the computer will boot up anyways. If it boots up and everything is working fine than your RAM is probably fine to use.
If it has problems booting up, Remove all the RAM modules EXCEPT one stick and see if it boots up.