3 head cassette decks

Akai. Still own one.

Back in the late eighties/early nineties I had a Teac V-800X that was quite good. During that time a local high-end shop had a Nakamichi rep come in with a promotional offer to perform a free tune-up and test on any deck as a means to compare to the Naks. After running tests on my Teac he seemed genuinely surprised at the Teac's performance. It was a surprise to me also as it measured better than its specs.
Unfortunately the Teac died. Maybe from lack of exercise.

Tandberg and Revox are already mentioned, as are Teac's. If you don't mind spending some serious money, the Teac Z-series are awesome. Also the better Technic's decks are quite capable, as is the Yamaha KX-1200.
Best for it's money I ever had was an Aiwa AD-F800: sturdy, easy to use and very good-sounding. Ultimately I prefered my Nak ZX-9, but for the price paid the Aiwa was unbeatable.

I still have a Yamaha K1000, but don't use it very much. It has 3 heads, 2 motor direct drives and was one of the few decks that has DBX. It still sounds great.

The NAD 6300 should be in the mix, an excellent machine.