In the US it is possible to buy simple 3-to-2-prong adapters for 50 cents or so at any hardware store. They all come with a little grounding tab that you are supposed to connect to a solid earth ground, but that nobody does, people just use them to plug 3-prong devices into 2-prong outlets.
This is an extremely bad idea as a long-term solution. That 3rd pin is there for a reason that has everything to do with safety. If you don't mind blowing up your laptop there's not a lot of personal danger since there is an intermediate stepdown transformer.
But this is functionally the same as clipping the grounding pin off the plug. Which means any stray voltage or circuit failure within the device will now send full mains power to ground any way it can, usually through a human being touching it. Have you even been shocked or "buzzed" by a guitar or a microphone? That's invariably the result of a an amp or preamp with a ground fault that was somewhere plugged into a circuit without a ground. If you had touched the same device while wet, or sweaty, or standing in a puddle you would have been full-blown electrocuted.
If you are a full-grown adult without a heart condition in the US (where mains power is 120-volt), and if you are in a building wired to modern electrical codes, you will usually survive such electrocution before the circuit-breakers kick in, maybe with some third-degree burns, permanent scarring, and nerve-damage in the extremities. Pets, toddlers, and people with heart murmurs may need immediate CPR but can sometimes be saved with instant medical attention.
If you are in a country with 230-volt mains, or if are in a US building without proper fusing/circuit-breakers, all bets are off. Being a path to ground in that scenario is easily enough to kill even a grown man.
Moreover, ground-lift solutions are illegal in the EU, where laws govern such things. The only reason that it is "legal" to lift safety grounds in the US is because the US does not have laws, per se, to govern such things. The US instead has a National Electrical Code which is not law but instead a sort of licencing rule. In effect, you can do whatever you want in the US, but your insurance will not cover the claim if you were lifting grounds and burn down your house or kill your kids. The US generally offers a more favorable legal environment when it comes to reckless pursuit of Darwin Awards, compared to other developed countries. In American practice, it's a matter of civil law: your lease, mortgage, insurance, etc all require you to follow code, but there is no government agency per se that regulates that such things, just the quasi-governmental NEC as well as UL ("Underwriter's Laboratories", the laboratory specified for testing such stuff).
That third prong is there for a reason. You used to be able to kill someone and make it look like an accident by tossing a radio or hair-dryer into their bath. But safety codes are always ruining things for clever murderers.
There are a variety of easy ways to solve the problem of ground-induced hum without turning your equipment into a wet-floor electrocution-machine. The common "ground lift" switch on modern guitar amps, for example, does not actually separate the chassis from the safety ground, it simply isolates the loop between pickups and preamps, and exposes you to no more current than what you generate by vibrating a metal string over a magnet. If you don't have that option, then simple "star grounding" (e.g. plugging everything into the same outlet via clever use of power strips) will typically neutralize any ground hum. If it does not, that's a pretty good indicator that throwing that device into a bathtub might be a good way to murder someone.
But disabling the "third prong" is a really bad idea. Almost anything it resolves can be resolved by simply plugging everything into the same power strip, or by plugging into a different outlet.
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