Been years since I have seen any 48" "Slimline" Lamps. (F48T12/SP/IS Lamps, that is!)
Only seen 2 Lamp Ballasts (Linear Magnetic Reactor type) driving each set of Lamps in an 8' 4 Lamp strip.
Never seen them (the strips) in a 3 Lamp (or 6 Lamp tandem) configuration.
Were the original Ballasts 3-Lamp Magnetic type, or a combination of 1 Single Lamp and 1 Two Lamp Ballast?
(the "combination" scheme reminds me of "Slave/Master" Troffer Fixtures).
I don't know if 3-Lamp Mag. Ballasts for I.S. are widely available - maybe a special order thing.
Some information on the Lamps / Ballast:
"Slimline" Lamps are Single Pin contact, Instant Start, cold cathode operation.
They can be "T8", "T10" or "T12" size, and range from 18" to 96" length.
The Lamp specification I listed earlier in this message, would be figured as:F48T12/SP/IS"F"
= Standard Fluorescent Lamp,"48"
= Apx. maximum rated Lamp Wattage (the actual wattage will be no higher than 80% of any nominal rated wattage),"T12"
= "Tubular" Lamp enclosure, diameter = 12/8" (or 1½"),"SP"
= Single Pin Contact (as opposed to Bipin or Recessed Bipin contact),"IS"
= Instant Start (as opposed to Rapid Start or prog. start methods).
As already mentioned, you may find an Electronic type Ballast of 3-4 Lamp output, which will have a compatable output rating and connectivity to the standard Linear Reactor (Magnetic) type Ballast, and use these for replacements.
The Ballasts' compatability listings and ratings should include driving (at least) 3 F48T12 Lamps - at the wattage rated by the Lamp Manufacturer.
Be sure the Ballast(s) are the Instant Start Parallel types.
If you get a 4 Lamp output type, simply cap the unused Lamp Lead.