You paint with a broad brush stroke. Your characterization of the signers as being in monolithic agreement over slavery is simply not accurate. A number of the signers were anti-slavery, including Roger Sherman and James Wilson, who proposed the three fifths compromise you reference (found in Article 1, Section 2, Paragraph 3 of the US Constitution) during the Constitutional Convention in 1787.
While on the surface, the 3/5 concept is degrading in that it considered a slave to be 3/5 of a person (which is an understandably offensive concept in our day and time), in reality this was a political compromise that reduced the representation of slave states by 40% relative to their slave population. Understood within its historical context, this was a significant step forward in terms of calling out the hypocrisy of those who wanted slaves to be counted fully in terms of a state's population and proportional representation in the US House of Representatives, and yet would not count slaves as men on an equal footing with free-men who were afforded the right to vote.
The compromise itself speaks to the ongoing tensions that existed over slavery, which were born out over subsequent decades of debate as the founders and those who followed them wrestled over the issue and its philosophical, political, religious, moral, social, and economic implications (good reference site addressing the abolition of the slave trade from the colonial US to the present is here:
http://abolition.nypl.org/home/)
The portion of the Declaration you quote is also significant in that it included the phrase "the pursuit of Happiness" -- prior statements, such as the "Right of the Colonists as Men" by Samuel Adams in 1772 spoke of Life, Liberty, and Property - also, John Locke's "Second Treatise on Government" from 1690 spoke of "life, health, liberty, or possessions." The exclusion of any reference to property or possessions is significant, and was a nod to those who opposed a statement that could have been construed as being sympathetic to the protection of a slave owner's right to view slaves as property.
To sum up, while you appear to be taking a "glass half empty" view of the Declaration, pointing out accurately the divide between the ideals of the Declaration and the reality of how they were applied, I'd prefer a "glass half full" approach.
The Declaration itself is a shining light - and the fact that it was crafted within such an imperfect historical context is all the more amazing. The language in the Declaration that "all men are created equal" in and of itself creates a tension, calling on future generations to justify how a practice that enslaves men and takes away their freedom can be reconciled with the self-evident declaration that all men have a God given and unalienable right to be free.
The light of this beacon was reflected nearly 200 years later, on the steps of the Lincoln memorial, by Martin Luther King, Jr., in an often overlooked portion of his "I Have a Dream" speech:
Hard to improve upon that...