In the future, it would help if you told us exactly what is confusing you about the sentence.
It’s in some sort of British English, and while I understood it right away, and recognize that it’s perfectly correct, it sounds rather odd to my American ear.
Sir: This is the person being addressed. We can ignore it in analyzing the sentence. That leaves us with:
You will scarcely credit that it took six warders to dislodge him, three pulling at each leg.
Scarcely: almost not, hardly
Credit: to believe (from Latin “credere”)
Warders: prison guards (“guards” in the latinized version of Germanic “warder”, via French)
Dislodge: synonym depends on context
Translation into standard non-British English:
You will hardly believe (or "you're not going to believe") that it took six guards to dislodge him, three pulling at each leg.
You will hardly believe X, Y.
X is a subordinate clause serving as a noun, the object of "believe". “You will hardly believe this fact. (This fact = “it took six guards to dislodge him.”)
Y is a participle phrase, serving as an adverb to give additional information as to how the guards dislodged the man. A lot of information is omitted, since it would just be repetitive. Replacing the omitted information, we get: “three guards were pulling at each leg” (in order to dislodge him).