It can get complicated unless you have 8 separate channels to keep the rear speakers, and a sound processor to bandpass the subs, at a different range of frequencies to keep 2 different subs playing the same frequencies, at least a 4 ch amplifier or 2 mono amps will also be needed to do this, and these amps should have an all pass, flat defeat crossovers option to make it easier to tune.
Not ideal, 2 better subs ran in stereo and one in mono.
The idea would be to run the factory sub from 80 and maybe up to 300hz or less and the other better subs from 20-30 up to -80hz, not good for the stage either but it may work with some extra tuning and phase shifting.
Instead of getting an inexpensive LOC, in order to this, just get a complete sound processor, it can take the hi level speaker outputs. Amplifiers need to be used, it gives you 8 channels to keep all channels and have full control of freq range per ch and even EQ per channel, output levels to balance and tune all channels
Some examples of popular DSP's would be JBL MS8, audison bit one or bit 10, Alpine PHX A series. PPI, soundstream, cadence and others offer more affordable DSP's.
Another option and way cheaper is to simply replace the factory head unit, with any of the 9 units available from pioneer, Alpine and the discontinued Clarion that offer bandpassing for the main channels, these will not have 8 channels available like the sound processors but you can eliminate the rear speakers, if you prefer to have 3 subs.
If no advanced sound processing is used 2 different subs will be playing the same frequencies, it will be loud and not play very good or play clean sound.