Now, to be clear, "can be sourced", includes the ability to source the required current, and still supply the requisite voltage required by what you are powering.
I also suggest you consider that there might be a "power up timing"
I've done something similar to keep from heating a thermistor by applying voltage from an output pin for a few mS while reading it, so your scheme will probably work, but, without seeing your wiring, I can't say for sure. Can you post a wiring diagram?
When all 3 optocouplers are on, you have your 3.3v (minus the voltage drop of the transistors) connected to GND through 3x 1kΩ in parallel. That draws 8 ... 10 mA. If your I/O pin can't source that, you could use a PNP transistor or P-mosfet on the high side. This is essentially a 3x1 input-matrix that you're scanning with a lot of dead time.
I suppose I would not have such a problem in the current scenario, am I right?
Should I use a digital or an analog pin set as output?
I connect 3 digital pin of my arduino to three optoisolators to detect some signals from my alarm system.The pins are in a pull-up configuration, connected to the 3.3 pin and to a 1 k resistor.
What if I change the +3.3 reference in the pull-up configuration, using another pin to act as the 3.3v ref?