The problem you have is a dog with a naturally high prey drive and hunt instinct. Beagles are massively different in character to dach's, they have a much higher intelligence and are bred to work so she's needing another outlet. Obviously you are doing your best by giving her 3 walks a day but it's very hard for her to have no outside space to be able to potter about in. You need to keep her mind occupied, play games with her inside the house. Use her natural ability to hunt and seek. Do you have an agility class nearby? She's too young to take part fully now but nothing is lost by letting her visit the classes and doing the less phyisically demanding parts.

When on your walks you can start teaching her to 'find', either hiding yourself or toys. It's a great distraction and works her mind as well as her body.

Found this online which seems to be useful. The Complete Idiot's Guide to Beagles - Google Book Search

Beagles are wonderful dogs but a bored beagle isn't good news and you need to cater to her breeding and adapt to her requirements.