From another perspective: The icons you're looking for are an integral part
of the Standard Toolbar, so they should already be available by default.
Although a button has been known to go AWOL from time to time it's unlikely
for all three to disappear at the same time. My guess is that you aren't
seeing the Standard Toolbar, itself.
Use the View> Toolbars menu to make sure Standard is checked. If it is and
you still don't see the bar [it includes New, Open, Save, etc.] then you're
most likely another victim of Apple's "minimize toolbar area" button ‹ the
oval device at the right end of the document window's Title Bar. Click that
button& see if it doesn't expose what you think is missing.
If the buttons are actually missing from the Standard Toolbar follow
Michel's direction to get into the Customize window. Rather than dragging
the buttons, first go to the Toolbars page, click the name of the Standard
Toolbar, then click the Reset button. If that doesn't restore them, follow
the remainder of his suggestion to replace the buttons on the bar.