I'll try and help.
1. Shikashi is use primarily in formal situations or in writing.
2. Kedo is short for keredomo. You can use it where ga could be used.
Eg.Kono kooto wa yasui desu ga shitsu ga warui.>Kono koto wa yasui desu kedo, shitsu ga warui desu.(This coat is cheap, but its quality is bad)
3. Dakedo is a contraction of dakeredomo, meaning "though/however". It's a more colloquial version of "desu kedo"
Eg. Oishii dakedo, amari kenkou ja nai. (It's delicious, however it's not very healthy)
I think that's the difference. If I'm incorrect, someone please correct me.
But: dakedo, kedo, and shikashi