At BA Longhaul a set of three high low chimes is an indication we are pulling onto the runway on shorthaul where there is no facility for these chimes the seatbelt sign is cycled to produce 3 chimes.
It's just a "final warning" signal to crew as the Senior will ask CC to take seats prior to informing the flight deck we are secure. We don't confirm this we are just expected to comply immediately.
When the flight deck call us you get a "high low high low" chime crew to crew is just "high low" and pax call bell is just a high chime.
Very uninteresting now you know. Sometimes fligth crew will "double ding" us if we have been warned about turbulent whether to let us know it is safe to leave our seats though this is agreed on the day as opposed to being an SOP.