Hawthorn is a small tree but can make a great hedge. To attain that they need to be planted very young ie about 1 - 2 ft tall, very closely and then cut regularly to make a solid bushy hedge. They will not grow bushy by themselves. As trees they are not very large and should need no pruning at all in tree form except to remove any dead, damaged or crossing branches. Cutting back branches will not improve the shape of the tree nor will it it convert a tree to a hedge. To do that you'd need to cut the tree down to a stump. The leaves are naturally small - about an inch to inch and half as long and even the healthiest specimen will not be a big dense tree. If you planted them at 10 feet they would only grow a further 10 - 15 feet or so to attain their maximum height. Most wild hawthorns here are shorter than that.

Why yours is particularly sparse in leaf I do not know but I understand they are not a tremendous tree choice for many parts of the US because it is susceptible to a number of diseases. Do you know which species and cultivar you have? Is it one of the pink ones? They are trickier than the straight Crataegus monogyna and laevigata which grow wild here and are seldom planted in gardens.

are they in full sun..

i usually attribute sparseness.. to lack of sufficient sun ...

and .. 3 years on a 10 foot transplant.. does NOT mean that they are fully established yet.. they may be.. or not.. but until they are fully established.. that might also cause sparseness...

an no.. in my book.. food or fert.. is not going to magically make anything happen.. unless a soil test shows something is lacking .. if you fert the lawn.. and they are near the lawn.. they are getting what they need.. no need to treat them to extra late night snacks ...

a little of this or that isnt going to hurt.. i just think it wont do anything other than make you feel better ... and dont waste your money on tree spikes ...

finally.. part of your problem.. is that i am thinking you are thinking about them like annuals and perennials .. and its your 'expectation' of things happening fast.. that is the root of your problem .. when dealing with trees.. you have to be thinking more along the lines of decades .. for success.. not a couple years .... which of course does nothing to solve the privacy screen ... anyway.. they are alive after 3 years.. you win..

cant go much further w/o a picture

ken

If they are C. monogyna this is a weed species in North America, quite prevalent in some agricultural areas.