I think that this is an excellent suggestion, and rather than trying to lock them out of the account or put it beyond reach, use it as an educational opportunity. Most 18-35 year olds today are aware of the challenges around mortgages/housing. Sitting them down and setting out a plan to get them on the ladder is an excellent idea. Explain that this is not a slush fund, drinks and party money, contribution to a shag-mobile etc and get their buy-in. Sure, they may wobble and mis-step along the way, but in addition to starting the monetary accumulation, planting the seed in the back of their minds about saving/deposit/mortgage sounds like a win-win. Some of the suggestions about putting it into an investment product, or a cumbersome yoke like state savings might put the money at one remove and discourage spur of the moment splurges. Bare trust etc sounds pretty ott. And definitely get them to commit to making some contribution, appropriate to their circumstances, no matter how small.I would make it conditional on them contributing a like or multiple of the 3k once they are able to avoid this.
Another option would be to put the offer on the table, ask them to think about it and come back to you in writing with a commitment about the ultimate use and keeping it at arms length in the meantime. It really depends on the personality you are dealing with, and the level of maturity.
The week I started working, my father sent a lad from Friends Provident to my desk at work with explicit instructions to get me to sign up for a "with profits endowment policy" for at least £10 per month (It was fado, fado, and this impacted heavily on my disposable income). He did such a good job selling I actually signed up for 2, one 10 years and one open ended. Years later these paid the deposit on my first mortgage in full.