Hello,
All Beograms will fit the Beomaster 3500. Technically there shlouldn't be any problems, other than they may be not visually matching. If the 3500 has the 1900 dimentions, you can use the Beogram 2400 series, Beogram 3000, 3500 of course,, Beogram 4500 Beogram 8500, Beogram 9500, even the Beogram 8000 series. All these models has the same slope as the Beomaster 1900/2400/2000/2300/3000 etc.
Corrections: The slope is probably the same, but the Beomaster doesn't have a built in RIAA, you have to use the Beogram 3500 or 4500 which has the RIAA fitted. Alternatively, you can fit an externally RIAA preamp and use other models. It is highly likely that the Beogram 3500/4500 is more expensive than their similar looking brethren.
-Søren