INorder to plug mics in the optical input, do I have to use a Hosa signal converter? If so, what's the difference between a Hosa 276a and a 312?
The JB3's optical input double as an 1/8" stereo analog input. So to answer your first question: you do not need a digital format converter to plug mics + battery box / preamp into the JB3's 1/8" analog input.
The Hosa ODL-312 and ODL-276 convert different digital formats to S/PDIF over optical cabling. The ODL-312 converts AES/EBU to S/PDIF over optical, while the ODL-276 converts S/PDIF over wire to S/PDIF over optical.
For the sake of discussion, let's make sure we distinguish between <1> digital format conversion (DFC) and <2> analog to digital conversion (ADC). Digital format conversion (like the Hosa ODL-xxx boxes) doesn't deal with analog signals at all. Digital format conversion simply converts one digital format to another. These DFCs are useful if one runs an external ADC that outputs a digital format different than the one required by the recorder's digital input. For example, the stock Lunatec V3 outputs S/PDIF over coax while the JB3 requires S/PDIF over optical. In this example, one might use a DFC - like the Hosa ODL-276 - that converts S/PDIF over coax to S/PDIF over optical. (One may also have the V3 modded to output S/PDIF over optical, but that's outside the scope of the example.)
Analog to digital conversion (ADC), on the other hand, converts the analog signal from your mics and battery box / preamp into the digital realm. The JB3 has an ADC built in, so technically you can just feed it an analog signal from your mics and battery box / preamp. In fact, I'd try that first and see if you like the results - if so, great. If not, then you can start looking at external ADCs.
What mics and battery box / preamp are you using? In what scenarios do you plan to record - open or stealth, loud rock & roll or unamplified acoustic or nature sounds or...?