Premature fruit dropping has many causes but in your case am certain that this is coming from an insufficient carbohydrate supply due to a lack of photosynthesis because the trees are in too much shade from the sycamore tree towering over them. It seems that you can do nothing about this situation, just accept that the friut treestree will not produce for you under these circumstances. If the other two trees are in full sun, then there could be a problem with the mineral content of your soil. The only other reason I could think of is inadequate water supply.

Most apples lose some fruit at this time of year - it is known as the June Drop. The tree knows how much fruit it can support to full term and as you say your trees are still young. If the pear is not too long established it might be worth the risk of moving it to a sunnier spot.