We sent a Tech Request a while ago and this is what Airbus said about this topic (320 family; there are other reasons to select gear down as per SOP on, say the 350, e.g., for the PLT to finish before landing):
The SOP philosophy regarding landing gear extension is to get it down and locked before reaching the final landing configuration. As either CONF FULL or CONF3 may be used for the landing, the SOP therefore requires selecting the landing gear down before selecting FLAPS 3, in order that landing gear extension is independent from the final selected landing configuration. This SOP recommendation was established to cope with the fact that the "L/G: GEAR NOT DOWN" red warning is triggered if the aircraft comes down to 750 ft radio height, with landing gear lever not in the down position. Indeed, if this SOP recommendation is not followed, i.e. if, for example, the landing gear and flaps 3 are selected simultaneously, and if the flaps/slats extension sequence is started a bit late, the landing gear selection may be close to this altitude threshold. Another point is that the L/G: GEAR NOT DOWNLOCKED warning is triggered if the landing gear extension sequence is not completed after 30 seconds: thus, selecting the L/G down after flaps 2 extension allow a comfortable reaction time, in the event that the warning is triggered. Finally, for each SLATS/FLAPS setting, the autotrim function trim the aircraft. The same applies when the landing gear is set to down. Waiting the landing gear is down locked before selecting Conf 3 allows getting the aircraft trimmed before a new SLATS/FLAPS configuration.
For all these reasons, we recommend to select the landing gear down before selecting FLAPS 3. However, there is no technical objection for selecting CONF 3 or CONF FULL before L/G extension sequence as long as the maximum speeds to operate SLATS/FLAPS and L/G are respected.