I think you're talking about three books, which I will attempt to identify.
First book 3 children brothers/sister exploring hillside and discover a cave entrance. They follow some type of path or stairs that go deep underground and discover some city of 'mole people'.
"Power of Three" by Diana Wynne Jones https://en.wikipedia.org/wiki/Power_of_Three_(novel), which features the relationship between two fantasy races (the Lyman and the Dorig) and humans (who the others refer to as Giants)
Lymen live in villages inside huge, hollow mounds. They fear the Giants and are at war with the Dorig. They operate their
magic/technology with Words. They are warlike and like hunting, but
they usually will not kill unless they have to. A few have special
attributes called Gifts which enable them to use a special magical
skill (e.g. see the future). Their clothes are the colour of the Moor,
so they can camouflage with the long grasses, shadows, and trees. They
look like humans, but are a little smaller, thinner, and have
almond-shaped eyes which are the same colour as a human's.**
Dorig live in halls under water. They have airlocks and working pumps.
They, too, fear the Giants and they are at war with the Lymen. The
Dorig have only the barest minimum of knowledge about Words and do not
seem to have Gifts, but can shapeshift. They are rather peaceful, but
are not reluctant to kill. Their real clothes are soft, and made of
fish-skins that are prepared to shift shape along with the Dorig that
is wearing them. When Dorig come to the surface, they don a hard,
scaly armour that covers them head to foot and also shifts shape along
The second book is "A Wrinkle in Time" by Madeleine L'Engle https://en.wikipedia.org/wiki/A_Wrinkle_in_Time
Other books, their father was some kind of scientist that was presumed dead? The children discover clues that their father was stuck in some alternate dimension?
One night, thirteen-year-old Meg Murry meets an eccentric new
neighbor, Mrs Whatsit, who refers to something called a tesseract. She
later finds out it is a scientific concept her father was working on
before his mysterious disappearance. The following day, Meg, her child
genius brother Charles, and fellow schoolmate Calvin visit Mrs
Whatsit's home, where the equally strange Mrs Who and the unseen voice
of Mrs Which promise to help Meg find and rescue her father.
Mrs Whatsit, Mrs Who, and Mrs Which turn out to be supernatural beings
who transport Meg, Charles Wallace, and Calvin O'Keefe through the
universe by means of a tesseract, a fifth-dimensional phenomenon
explained as folding the fabric of space and time; this form of travel
is called tessering.
The third book may be the sequel to Wrinkle: "A Wind in the Door" https://en.wikipedia.org/wiki/A_Wind_in_the_Door
There was also something about an "X"; it was some kind of symbolic clue.
The Echthroi are powerful, evil creatures whose desire is to X (i.e.
extinguish, unname) creation.
All three books would have been available in the mid 1970s.