I haven't tried it myself but I have read that a great way to fill tall raised beds is by making them hugelkultur beds. Some people make hugelkultur beds several feet tall.

+1 on hugelkultur,also take a look at "keyhole gardening". I'm unable to bend at the waist so I must get on hands and knees to plant,weed and hearvest garden beds. I fashioned a grid from 3/8ths rebar to go in bottom of a hot tub. The grid hold's logs 8"above bottom where water that run's through can be drained to prevent waterlogging soil. I started at grid with 6" to 8"logs and built up with progressively smaller material until leaves were covered by garden soil. It was very productive the first year,this is the 4th year and I love the convenience of standing to tend plants. In addition to mulch,I add 2 or 3 inches of soil each year to compensate for settling as material rot's. I've been building conventional hugelkultur mounds around the place where I will plant grapes,berrys and fruit trees eventually. Without walls,I don't think Lasagna 3 feet high could be kept compact over time. I believe one could use 2x lumber,4x8 concrete block or stack rot resistant logs like Cypress or Eastern Red Cedar,drill and pin them with rebar/large nails to create a long bed such as you want.