If you're going to go directly from bottles to a 3L starter, you'll need a few cases to properly seed the starter. Probably better to start small and step it up.
For my 2 hearted clone, I use 3 cans of Oberon. I do a 0.25L starter of 1.020 for a couple days, add 0.5L of 1.030 wort for another couple days, then 2.0L of 1.040 wort. For Hop slam I would replace my 2.0L with a 3.0L.