OK. If you're running non-clutched 10-sp. swap your rear derailleur with a clutched type, Shadow+ for Shimano, Type 2 or 2.1 for SRAM. If you're running 9-sp. SRAM there are bodge options using Shimano 10 or 11-sp, or SRAM 11-sp derailleurs to SRAM shifters, but if you're on Shimano 9-sp. you're stuck with it unless you do a 10 or 11-sp. upgrade which at minimum requires cassette, chain, right shifter & rear derailleur.
There are under-chainring guides that mount to the bottom bracket (take the spacer out from behind the r/h cup to fit) which tame the chain slap to an extent, but I've only ever seen them for single & 2x, not 3x.