Stereo Bus Summing To Mono

You need 3 busses:
 

1. Strings
2. Reverb
3. Master

 
Set the OUTPUTS of your tracks to the Strings buss
Insert a SEND from your tracks to the Reverb buss
Set the OUTPUTS of the STRINGS buss and REVERB to buss to the MASTER buss
 
Insert your reverb of choice in the Fx bin of the REVERB buss. make it 100% WET or if that option doesn't exist, kill the DRY part.
 
Adjust your track SENDS to taste
 
All these busses are stereo.

To check if an effect is forcing mono summing, it's not enough to bypass the effect. You have to actually remove it from the fx bin. 
 
Most reverbs do sum to mono, but some are "true stereo", meaning separate left/right reverb engines. Contrary to popular belief, though, these do not necessarily create more natural-sounding reverb. That's because real acoustical reverberation doesn't work that way - in a real space, delayed versions of each instrument are going to be all jumbled together and hitting your ear from all angles. Only the original direct signal is going to have a well-defined point of origin.
 
Here's what you want to do...
 
1. Create a bus called STRINGS and route each of the three string tracks to it. Don't put any effects on this bus for now, although you might want to add an EQ or compressor later on. But no reverb. This bus is just for setting the level of your strings as a group. Route this bus directly to the master.
 
2. Create a bus called REVERB and insert your reverb plugin there. Set it to 100% Wet. Also route this bus to the master.
 
3. Create AUX SENDS on each of the string tracks and route each of them to the REVERB bus. 
 
That's it. Your string sections should preserve their pan positions. The reverb bus, however, will be blurred panoramically, as it should be. Use the REVERB bus' volume to set the overall reverb amount.

Pardon me for cluttering this up.  I'm a curious sort.
 
What are the pros/cons to using a send from the STRINGS bus to the REVERB bus instead of individual sends from the each of the string tracks?

I typically send different levels for each string section. Violins get the most reverb, violas slightly less, cellos much less and basses almost none. This isn't done in order to enhance realism, though, which it obviously doesn't. It is done to enhance clarity. 

If there were just a small number of instruments and nothing else eg a string trio there is also some merit in sending the buss mix into a reverb also.  You can end up with a more balanced reverb over all three instead.
 
I patch in an EQ before the reverb and it is here that you can actually control what goes into the reverb a lot more.  The bassy instruments can have a HPF in action and that will limit the low end that goes in there and gets muddy.  All three instruments in a way should be in the reverb and if they are, the reverb will sound more interesting and complex instead of just being on one higher pitched lead instrument.
 
Having the violin only say getting the reverb and bass none might make things sound a little strange.  The upper harmonics of the bass can still get in there using the pre EQ but muddiness can still be kept low.  If there is a powerhouse rhythm section going on at the same time though then using individual sends could also sound better.  Depends on what you are trying to achieve.   A nice string trio in a nice small sounding room? 

I would definitely try for some sort of stereo final image. You have got three sources which can be positioned and then the reverb can be added. Early reflections as well as reverb components.
 
One thing is I have invested in an amazing plugin called Panagement. It is here:
 
https://www.auburnsounds.com/products/Panagement.html
 
You use this conunction with your current reverb. In my case that happens to be Liquid Sonics Reverberate V2. This plugin allows you to do something much more easily then just using a reverb plugin on its own, and that is to give a full 3D effect in a way allowing not only panning from left to right but front to back as well.
 
With a string trio like this you can use this plug-in to position one instrument to the left, the other towards the right and the third in the centre but a little further back.  All three end up in this amazing 3D like space. You can actually move all three around like this.  It can sound uncanny and so real it is very impressive.
 
The trick is to select and edit a real nice stage or room reverb and just paint that in small doses then the whole thing sounds like it is on a stage somewhere and has real depth to it.  Keeping and EQ patched in before the reverb still all applies as well.