(This is from Kerbal Space Program 0.25 with the RemoteTech-v1.5.1 mod.  One of the requirements of Remote Tech is that unmanned probes have a two-way communication link back to the mission control at Kerbin Space Center.)

Currently, I have (5) satellites in stable orbits around Kerbin.  The range on the "Communotron 16" is 2500km and Kerbin has a radius of 600km.  In order for satellite A to talk to satellite B, the orbital diameter needs to be under 2500km (worst-case).  2500km / 2 = 1250km radius, minus the 600km radius of Kerbin puts orbital altitude at 650km.

Note: Since Kerbin blocks line of sight (LoS), the orbits can be slightly larger then 650km.

The other way to estimate the correct orbital distance is to find the orbit whose circumference is around (N x 2500km).  Assuming three satellites and a circumference of (2500 x 3), this gives a radius of 1190km, or an altitude of 590km above the surface of Kerbin.  This is a very naive calculation and the orbit can actually be larger

In the case of an equilateral triangle where all three satellites are evenly placed around Kerbin, you can calculate the radius as (N / sqrt(3)) where N is 2500km.  That gives an orbital radius of 1443km (843km altitude).

So the best range for (3) equatorial satellites is 590-843km above the surface.  It is probably best to shoot for 750-800km orbit altitude which gives you a margin of error.  The assumption here is that we use the 2500km omnidirectional antenna to talk to KSC and the DTS-M1 directional links to talk to the (2) sister satellites with the 3rd link being used to talk to the "active" ship.

Update: I am going to go with 700km altitude orbit and (4) satellites on the equator.  They can be setup roughly equidistant from each other and can all communicate using the 2.5Mm omnidirectional Comm-16 antenna.

Hermes I -  In 700km altitude orbit.

2210 EC 1.78 EC/s (36) OX-STAT PV Panels

DTS-M1 #1 Active Vessel
DTS-M1 #2 (OFF)
DTS-M1 #3 Hermes III-B

For running all (3) Comms DTS-M1 antennas, power drain is 2.6 EC/s which requires 1999 EC reserve power for the 769s orbit dark period at 750km.  By tightening up the orbit to only be 700km, we only need 1968 EC reserve power for the transit.

Hermes II-A - Nearly perfect 700km orbit.

2210 EC 1.78 EC/s (36) OX-STAT PV Panels

DTS-M1 #1 Active Vessel
DTS-M1 #2 (OFF)
DTS-M1 #3 Hermes III-B

Hermes II-B - Orbit is approximately 700km altitude.

2210 EC 1.78 EC/s (36) OX-STAT PV Panels

DTS-M1 #1 Active Vessel
DTS-M1 #2 (OFF)
DTS-M1 #3 Hermes III-B

Hermes III-A - Target 12.676Mm orbit (just past Mun)

2210 EC 1.78 EC/s (36) OX-STAT PV Panels

DTS-M1 #1
DTS-M1 #2
DTS-M1 #3

DV Fuel: 3644 m/s (~55% full), Mass (current): 2848kg

Highly elliptical orbit (10188 km Ap 303km Pe).  Currently having communication difficulties back to KSP which prevents the Ap burn to circularize the orbit and raise the Ap to 12.7Mm.

Hermes III-B - Needs minor orbit tuning

2210 EC 1.78 EC/s (36) OX-STAT PV Panels

DTS-M1 #1 Active Vessel
DTS-M1 #2 Mission Control
DTS-M1 #3 (DO NOT USE)

DV Fuel: 3354 m/s (~50% full), Mass (current): 2641kg

Synchronous orbit approximately above KSC.  Current orbit is 2846km Pe / 2886km Ap.  A perfect synchronous orbit is 2868.75 km.  Minimum required battery for orbital dark period (1195s) is 2128 EC.  There is not enough power to run a 3rd DTS-M1 link during the dark period.