Hello and welcome to Beoworld.
Sad to hear the passing of your grandfather, though congratulations for getting such a fine system.
The Beomaster 3000 is a receiver (a tuner with built in amplifier) to which you can connect speakers. It is a very reliable unit, being of a very good design. The 3000 pumps 2*30 watts PC out and 2*40 for the newer 3000-2, although the difference between the two is really small.
If you would like to connect your Beosystem 3000 to your Bose system, you can use the headphone output of the Beomaster to feed to the input of the Bose. Just use a regular Stereo-jack -> dual RCA plug cable, but I would suggest you to find a pair of matching speakers and make a whole different setup. A good speaker for the Beomaster could be the S-45 (old version and -2 version), S-40 or S-35. They are all with rubber sourrounds, that means that you won't have a problem with rotten woofer sourrounds now or later on. All three speakers did come in Beomaster 3000 standard wood veneer, Rosewood, Teak and Oak and they sound very good and balanced.
The Beomaster can form the base of a new system where you even can connect your iPod and computer to, with the help of 5-pin DIN connectors to use on the tape source plugs.
Oh, forgot about two things. There are two "phono" inputs, the second one is at line level so you need to plug your Beogram into the "phono 1" socket. Your Beomaster 3000 may be, as Evan said, the newer model with touch sensitive buttons.
Links for the two:
Original 3000: http://beocentral.com/beomaster3000-1970s
3000 from the 80's: http://beocentral.com/beomaster3000-1980s
Enjoy your trip into B&O, I'll swear that it's going to be addictive if you like the Beosystem 3000!
Beocenter 9300, Beogam CD50, Beocord 5500, Beomaster 3400, Beomaster 4400, 2 Beogram 4000, Beomaster 8000, 2 beogram 8002, Beovox S-75, Beovox MS150.2, Beovox RL6000, Beovox S-35, Beomaster 6000, 2 Beocord 9000, Beocord 8004, Beocord 5000, Form 1, 2x Beolink 1000, Beo4, MX3500, LS4500. Born 1993.