R.D.
Well, since you HAVE to do this, just prepare them. Get them excited about it! Consolidate their stuff; get rid of the furniture you won't be able to fit. I've been there. How much do you trust your 3 year old? My kids were able the same age and when they had to share a room, I bought bunk beds so they had more room to play. The daughter that was 2 1/2 at the time was a monkey/ninja, so I totally felt comfortable with her on the top bunk. The 'baby' had been in a toddler best since 10 months (they're both climbers) so she had the bottom bunk. I took away the ladder... the older one didn't need it (again, MONKEY) and I didn't want the younger one even attempting to get up there. Now, the girls are 5 and 7 and we still have that bed! Best decision I've ever made.... and like I said, it cleared up a lot of room so they can actually put all their toys in there and PLAY :)