Each of these documents arose from a Congress and every colony/state sent representatives to the Congress. Sherman represented Connecticut in virtually all colonial matters from 1774 until his death in 1793. Before being chosen as a delegate to the Continental Congress he served as Mayor of New Haven CT.

Morris helped finance the Revolutionary War and was elected as a delegate from Pennsylvania for the Second Continental Congress.

The Declaration was signed during the Second Continental Congress which lasted from 1775-1781. The Declaration was signed in July (or August depending who you trust) 1776 but the Articles of Confederation weren't ratified until March 1781. The colonies were free to change their delegates and many did. You have to remember that it would have been a very long and arduous journey for what were mostly older men already financially secure doing this out of a sense of duty. It was appropriate to send a respected man but that same man might be reluctant to go multiple times over a six year span.

As for the Constitution that wasn't signed until 1787. From the signing of the Declaration in 1776 to the signing of the Constitution 11 years went by. It's actually pretty amazing that two men signed all three documents, not that there weren't more who signed all three.

There were also other factors that contributed to the situation. For example, Benjamin Franklin signed the Declaration and the Constitution but did not sign the Articles of Confederation because, despite writing most of it, he was in France negotiating for the US. He surely would have signed had he been in the country.

Washington was present at the signing of all three documents but was not the delegate from Virginia and was present for his expertise. He only signed the Constitution as the President and deputy from VA. Neither Jefferson nor Adams were available to sign the Constitution either as Jefferson was in Paris as well and Adams was in Europe serving as minister to Great Britain.

Samuel Adams, Patrick Henry and John Hancock, both older by the time the Constitution was being drafted, turned down opportunities to affix their names to the document because they feared it would lead to another monarchy.

In the end there were 70 delegates to the convention that signed the Constitution, only 55 were able to attend and only 39 of them signed the final document. Many of them were just different delegates than had been there for the Declaration and the Articles of Confederation.