1) Sources A, B and C are war recruitment posters published by various governments with the aim of influencing more people to volunteer for armed service in the war.
Sources A and B are an earlier type of source, depending on the patriotic fervour that swept Britain at the war's outset, portraying enlistment as a duty to the country and empire. The posters themselves being of an accusatory nature, demanding from the reader "What did you do in the war?" and that they should "Go!", the fighting taking a crusade-like facade in which the only way to please parents, friends and girls was to join up and head towards the fighting. That this was accepted by many, was partly because the war was seen as an adventure, and perhaps because the last war where there was mass recruitment was almost one hundred years previous, the majority of those fighting in the interlude being well-trained career soldiers.
Source C, however, is a much later source, as can be determined from the approach it uses to "persuade" people to enlist, preventing the "mad brute (of) militarism", in this case, a raving gorilla, that represents Germany, from reaching out from Europe (bottom-centre, right) which has been decimated, to the shores of "America" (bottom, centre). The poster compels the reader to joint up for the US army, probably after the USA declared war on Germany, on the 7th April 1917, proving this to be a later source, produced after the war has raged unabated for three years, thus having dispelled the notion of adventure or even perhaps duty.
2) The three posters are very different in the fashion their aims are laid out, nevertheless, there are some similarities between them. The earlier sources provoke Victorian attitudes towards duty, leading to the enrolling of over two million volunteer troops into the British armed services from 1914 - 1916, while source C plays on the moral decency of the reader, calling for a halt to the foul deeds committed by the ape (representing Germany) that has caused the desolation of massive amounts of territory, together with the atrocities committed by soldiers under the direct control of Germany.