While there are adjustable linear voltage regulators which can deliver up to 50 Volts, that is probably not what you are looking for. At the voltage and current ratings required, a linear regulator would need to dissipate a lot of power as heat, since presumably the regulator would be fed off a bridge-rectified output from a transformer, and efficiency for a linear regulator is not very high.
This drives the linear regulator approach out of the realms of practicality.
A better option is a switched mode / buck regulator. These generate less wasted energy or heat, due to their very high efficiency, typically between 80 and 95%. A few suitable parts do come up on a product search.
Taking this approach a step further, switched mode power boards for 50 Volts / 3 Amperes can be designed using free online tools such as TI's WeBench. This would use a buck controller (e.g. LM5116, adjustable up to 80 Volts) rather than a regulator, external MOSFETs, and other components for a complete device such as below:
Even so, the solution remains sub-optimal as you would require a fairly large and heavy step-down transformer to bring your mains voltage down to the required levels, and to isolate the output from the power lines for safety.
Next option, an SMPS:
Mains-powered SMPS units of voltages and currents close to your requirements are available off the shelf from vendors specializing in industrial power supplies. Failing this, SMPS manufacturers can custom-build an SMPS to your required rating. I have recently had an SMPS somewhat like in the question custom built by a local manufacturer, and it worked out not nearly as expensive as I had expected.
On another note: Many DIY audio amplifiers use unregulated power supplies, consisting of a step-down transformer, a full bridge rectifier and large reservoir capacitors, for driving the power amplification stage in balanced / dual rail mode. This will typically cost much less than a custom built SMPS, and might serve the purpose with some constraints.
From a cursory search on digikey.com it appears that regulators and buck controllers for the required voltage and current ratings in your question, are all surface mount packages. Assumption: You are comfortable using SMD components in your design.
