If a US president, through executive order, has the power to create monuments, logically, a subsequent president, through executive order, has the power to alter or even negate the creation.
Any president's executive orders last only until subsequent presidents alter or repeal them. Supporters of President Trump's executive orders or Trump's repeal or alteration of President Obama's executive orders should remember that. Trump's time will pass.
If people want these three monuments left unaltered, they need to contact their US congresspeople and ask they they be enshrined in a congressional act.