I have read some of Lillian Too books Too books and some of which are about the three legged toad and i have not read any activation for it, you just have to display it. Some of the lucky charm vendors I spoke with in china town told me that the coin in the mouth of the toad is very important-I think a three legged toad would not be effective if it does not have a coin in the mouth
To cleanse the charms, most of the vendors I see put the item in the middle of the singing bowl and have the sound cleanse it. I have heard and read some do come to temples to be blessed by the Taoist priest/monk or other religious clergy. Some cleanse the charms by incense smoke.
In some cases, I also read some articles that you "dot" the eyes of some figurines like dragons,piyao's, chilin etc by a red ink or paint to make it energized. Some would tie a red ribbon to activate it.
These are some of the things I know about your question. I hope I was of great help.