Usually a 30V, 3A lab bench power supply is capped to those specs, so you don't get 10A at 9V.
However, some of them do have 2 or 3 or more configurable ranges. Each range works like the normal way. For example, a 90W supply might have a 30V, 3A range and a 15V, 6A range. When set to the 15V range, you can get up to 6A, but you don't magically get 9A at 10V. The limit is still 6A when in that mode.
PC power supplies are essentially free and can provide tremendous amounts of power, but should only be used with great caution. Many are designed with an assumption of substantial current from the 12V line. Some will lose regulation and output terrible noise or 7+ volts on the 5V line if the 12V lines are unloaded. Beware! Several people have killed their Teensy 3.0 using a PC power supply 5V output with the 12V line unloaded. You may need to mount a 20 or 10 ohm, high power resistor in the fan's air flow, connected to the 12V power. Check the 5V and 3.3V lines for correct voltage and low noise before using them for Teensy!
The main advantage to lab bench power supplies is the adjustable current limit. You simply don't get that from a PC or any other normal power supply. If you set the current limit low, it will often save your parts from destruction if you make a mistake. For example, while working with a Teensy and perhaps a few LEDs, you might set the current to 50 mA max. Even though the power supply could be set to 3000 mA, if you accidentally connect the 3.3V power backwards to your Teensy 3.3V, the maximum 50 mA will flow through the P-N junction that forms the body of the chip. The power supply will automatically reduce its output voltage, so no more than the current limit can flow. Normally Teensy can survive 50 mA of reverse power current. 3 amps would be a "magic smoke" situation. Likewise for connecting transistors and other parts, the adjustable current limit, if you set it only slightly above the current you really need, will spare you from burning parts by flowing the full 10 to 20 amps or more a PC power supply would output.
But a decent lab bench power supply is at least $100, which is a lot of money if you're doing electronics on a student or hobby budget. Still, after a decent temperature regulated soldering iron, a quality power supply with working current limit is one of the best buys to make fiddling with electronics much easier. Everybody makes mistakes, no matter how experienced, and especially when you're just starting, a current limit set relatively low will save you many times from accidentally destroying parts.
