I still have some Hornby Dublo 3 rail and A3 and A2 controllers, though the locos rarely get used unless I take them to a club or other organisation which has a layout.
When I went 2 rail I bought Hornby Dublo locos (identical to the 3 rail ones apart from different numbers, 80033 instead of 80054 for example) and ran them on Peco track using H&M controllers (duette and similar) with no problems. I sometimes take the 2-rail locos to local clubs and they seem to run fine on most controllers without any form of overload indication. Problems, or very slow running, are usually a sign that the loco needs to be cleaned and oiled or that the magnet needs attention.
If you want to be authentic and use an A3 controller they can sometimes be found at swapmeets and toyfairs, together with many H&M controllers. I've also come across them in sales cabinets at model railways at preserved lines.
A3's can be found on the internet but at prices I certainly wouldn't be prepared to pay. You can even find A3 spares (e.g. bulb and its cover)!
