how to run three subs from one lfe jack

One sub from the receiver

Two subs from 2 speakers  

https://www.residentialsystems.com/technology/audio/svs-debuts-soundpath-speaker-level-subwoofer-adapter

I am sure there is something in the pro world for running powered speakers that would work. Maybe call sweet water and ask them.

Some active crossovers will let you run 3 subs with active gain output (preamp). Sublime acoustics is a good example. Two subs could run off the RCA outs and one off the XLR as both are active at the same time.

For background music I would just use a Y adaptor or two and turn up the receivers sub gain 90% and then adjust it down in the subs as needed. 

Audiotroy call audiokinesis.

Rolls DA134 Line distribution amplifier

SPECIFICATIONS
The DA134 is a four channel RCA distribution amplifier with RCA and
1/8” inputs that can be switched from mono or stereo. The DA134
Distribution Amplifier will distribute a mono signal to eight outputs,
or a stereo signal to four stereo outputs. The outputs are designed to
drive line level signals. The DA134 is the perfect distribution amp
anywhere high quality sound distribution is required.
Input Impedance: 10 KΩ
Max Input Level: +12 dBV
Output Impedance: 470 Ω
Max Output Level: +12 dB
Max Gain: 20 dB
S/N Ratio: >90 dB
Cross Talk: -70 dB (stereo)
Frequency Response: DC - 60 kHz, +/- 1 dB
THD: <.008%
Power: 15 VDC
Size: 6.5” x 3.5” x 1.5”