Pruning suckers from the base of a sapling pin oak
I have three large suckers coming from the base of my sapling pin oak. Because the suckers are so closely aligned with the main trunk the pruning cuts will be long (and difficult). I worry that these cuts will weaken the tree. I have attached three photos. Any advice you can offer would be greatly appreciated.
The additional trunks may have developed in response to a past injury to the sapling's crown. As you have concluded, they must be removed to permit normal growth of the main stem.
Because the extra stems comprise a sizeable portion of the plant's mass, it's best to remove them in stages. Remove the branch that's farthest left in the photo now. Thin and shorten the other two on that side but let them grow to nourish the tree this summer. Thin and shorten the excess stem to the right somewhat too.
Next winter remove the remaining excess branches to the left with a sharp saw cutting them off close to the main trunk. Thin and shorten the excess stem to the right again at the same time.
Finally, two winters hence, cut off what remains of the excess stem to the right close to the main trunk with a sharp saw.
Because it's difficult to determine the best way to proceed based solely on viewing the photos, you may want to consider asking a certified arborist to take a look at the tree onsite and do the work for you or, if needed, modify the advice we have given here. The following information may be useful if you decide to hire an arborist:
How to hire a professional arborist to help care for your landscape trees