Personal risk tolerance could also be that the house is a 3+ year investment and hopefully it will appreciate while at teh same time provide you a stable shelter.
Remember, there is no absolute guarantee that the market will produce gains the next 3 years,
but weigh that against the possibility of you put the huge down payment in to the house, and house loses a bit of value over the next 3 years.
Would you be stuck at the location? Would this house now become a crutch on your freedom locking you down in one place, or would it become an additional liability that needs to be paid down for the next 20+ years while you're no longer there. Some people are unable to cut their losses, others can't stand such a chain on their freedom to up and change locations.
Take some time and consider what can happen, and what you would do in each situation (it's easier to do it now while you're free of the emotions). If you can plan for it, then stick to that plan, there will be fewer surprises.