Hello,
it really depends on how much room there would be between the pine and the other two trees at maturity. It’s a common mistake to plant young trees too close together only to have them grow into each other and eventually get too crowded,

I don’t have any personal experience with this variety of white pine, but the information I found at several websites that sell this plant indicate full sun. Both silver maple and blue spruce can be very broad trees at maturity; a silver maple canopy can be up to 35-50’ spread and blue spruce can be 10-20’ spread with the widest part at the bottom. Stowe’s pillar white pine is listed as having a 10’ spread at maturity, so to give all three trees ample room you would ideally want to have 30-40’ between the trunks of the two existing trees. This might also be achieved by offsetting the white pine somewhat, if possible, so that it’s not exactly between the two existing trees.

I’m including an information sheet from the American conifer society to give you more information about this variety. One item to note is that to maintain the strictly columnar shape of this tree, it seems that it needs yearly pruning throughout its life to maintain the tight growth habit.

https://conifersociety.org/conifers/pinus-strobus-stowe-pillar/

A few shade tolerant small trees and shrubs you might consider are, flowering dogwood, redbud, rhododendron, silky dogwood, and spicebush. 

You'll also want to consider your soil type and moisture level (does the soil stay consistently wet, or dry out fairly quickly, or something in between).

Good luck!