As a result of the LHC injectors upgrade after the second Long Shutdown (LS2), the expected Pb-Pb luminosity and collision rate during Runs 3 and 4 will considerably exceed the design parameters for several of the key ALICE detector systems including the forward trigger detectors. Fast Interaction Trigger (FIT) will be the primary forward trigger, luminosity, and collision time measurement detector. It will also determine multiplicity, centrality, and reaction plane of heavy ion collisions. FIT is expected to match and even exceed the functionality and performance secured during LHC Run1 and Run2 by three ALICE sub-detectors: the time zero detector (T0), the VZERO system (V0), and the Forward Multiplicity Detector (FMD). FIT consists of two arrays of Cherenkov radiators with Multichannel Plate - Photomultiplier Tube (MCP-PMT) sensors and of a single, large-size scintillator ring. Because of the presence of the muon spectrometer, the placement of the FIT arrays is asymmetric: ~800 mm from the interaction point (IP) on the absorber side and ~3200 mm from IP on the opposite side.