Avoid scam sites that ask for credit card information for a "instant download." The M-2600 MKII manual is abandonware—it should be free or very low cost (under $5 for a high-quality scan).
According to the manual, when you want to record a guitar onto Track 3: tascam m-2600 mkii manual
Switchable -10dBV or +4dBm signal levels for Tape In and Group/Direct Outputs, allowing for easier integration with various recorders. Avoid scam sites that ask for credit card
Tascam M-2600 MKII Go to product viewer dialog for this item. tascam m-2600 mkii manual