The numbers I am giving are on average and not exact.
Orange Led = 2v drop, 25mA (.025)
You are using 3v so subtract 2v from 3v which gives you 1v.
Now to find the resistor value divide 1v by your Led current of 25mA or .025 = 40R resistor - You wont find the 40R so use a 39R that's the closest.
Something to consider...... If you wire these in series you only need one resistor but with only a 3v supply you will have dim Led's just by adding a couple together. You can wire them in parallel but then you need 1 resistor for each Led.
The answer to the second part is yes you can flash the white light but could you give more detail such as the size of the Led's 3mm, 5mm, 10mm.... Do you have the part numbers for the Leds? That way we could look up the datasheets and get exact numbers. If you could please draw a simple schematic or picture of some sort showing exactly what you are doing and with how many total Led's.