I have one of these: Trimetals bicycle storage

They're not cheap, but if you pay the extra for the four lock version and buy four very solid padlocks - I went to a locksmith and got four locks pinned for the same key for ease - I'm sure you'll find it strong enough for the front garden.

Mine is secured to the concrete below it with eight Rawbolts, so it can't be lifted and anyone who wants to get inside has four locks to cut off. I'm sure it could be done but it would involve some tools, time and a lot of noise. Not something that your average burgalar is going to want to do, especially in a front garden.

It holds three bike easily, four at a push, if you twist the handlebars.