Very Concerning Perforation in Hornbeam Bark
In general, the trees look stressed and may not be a good tree for this landscaped area. It often does best in a naturalized setting.
There is bark splitting and we notice included bark in the middle photo. The bark splitting is due to a type of stress. Stress can be due to planting too deeply (you should be able to see the flare at the base of the trunk where it joins the root system), excessive mulch, poor drainage, drought, possible borers, etc. See our publication on these types of problems https://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/hgic/HGIC_Pubs/TreesandShrubs...
Included bark is bark pinched between two stems creating a weak union. Co-dominant stems with a ‘V’ shaped union are often accompanied by included bark. This union is weak because the bark inclusion prevents any physical connection between the two stems. Instead of overlapping wood creating a strong connection, the two stems push each other apart as they grow and a crack develops. Proper pruning when young can help to prevent this. See more https://hort.ifas.ufl.edu/woody/bark-inclusions.shtml
There is no treatment for the above. All you can do is water during dry periods and make sure mulch is no thicker than several inches and keep away from the base of the trunk. Eventually you may have to replace the trees.