50 volts regulated power supply
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.