When saying the full rosary in three separate parts throughout the day, according to the distinct sets of mysteries, is it necessary to begin with the Apostle's Creed each time after it has been said for the first set of mysteries?

Also, is it necessary to pray the initial Our Father and three Hail Marys and conclude the prayers with the Salve Regina?

I realise that if I said the entire Rosary in one go it would obviously only be necessary to say the Creed and Salve Regina once, but I'm just not sure what the procedure is for when saying three partial Rosaries throughout the day?

You can say it either way. Once at the beginning of the day for all 15 mysteries should be fine. 

I am fortunate to have the time to say all 15 decades most days of the week and I break them up into the 3 sets of mysteries the majority of the time. I personally say the beginning prayers and ending prayers with each set of mysteries. If I pray all 15 decades or if I pray two sets of mysteries at once, I only say them at the beginning and end.

Whether praying the full Rosary all at once or pausing during the day, one way to maintain continuity of focus could be to note some of the prefiguring going on among the Mysteries. For example, in the fourth Joyful Mystery there's the prophecy of Simeon (Luke 2:34-35); in the fifth Joyful Mystery the three days apart point to the three days in the tomb before the Resurrection (i.e., the first Glorious Mystery). 

Also using mental imagery, perhaps consider together a painting or sculpture of Mother and Child together with one of the Pieta. (For example, during Holy Week, we might see these in printed material or online spliced side-by-side as mirror images.) Too, there are depictions in art that show Christ during the Assumption and Crowning. Contemplating these juxtapositions can help to remind us of the Marian elements of how the Mysteries tie together.

We pray a rosary as a family in the evening with the introduction and 5 decades.  Before this, throughout the day, I'll pray the other mysteries, but without the Creed or introductory Our Father and Hail Mary's.