My guess would be that excessive voltage killed them, which fits with "very bright". If that idea is correct the problem would only occur beyond a critical speed.
The 6V/3W rating of the dynamo applies only to the specific load presented by the original bulbs, otherwise much higher values are possible and can be measured. The bulb replacements likely draw constant power: as speed rises, so will the voltage and the current will fall, causing a further rise in voltage ... Wiring in series would probably help, but will reduce the amount of light at low speed, raise the starting speed ,and does not really solve the problem.
As suggested above, voltage regulation with Zener diodes is probably the simplest solution. A pair of wire-ended (through-hole) diodes rated at 7.5-12 volts (12 for maximum output) and at least 1W should be connected back to back (there is normally a stripe to indicate polarity) across the input, downstream from the switch. If lacking soldering tools, screw down terminal strip should work. These parts are cheap, about 25 cents even from the big distributors like Mouser. I have not actually done this, but have used a similar arrangement with a self-built LED bulb replacement for about 10 years.
Alternatively, there is a UK Ebay vendor, bikeco_uk selling ready-made regulators for this. They also have some bulb replacements specifically for dynamo lights, but do not say what is different.