You can start it today with no problems. Using
You need about a liter starter. So, you'll be doing a little less since you only have a 1000ml flask. Good rule of thumb I use for starters is: 1/2cup DME for each pint of water. A pint is close enough to 500 ml for me. Make a 750 ml starter with 3/4 cup of DME, put some sanitized aluminium foil on top and shake/swirl everytime you walk by. Since your only doing 3 gallons I would put it in the fridge on Friday (if it's done of course) and let it chill in the fridge til Saturday morning. You should be able to decant most of the starter beer and just leave your yeast behind.