Crux Spes Unica

"Crux Spes Unica", which is the motto of the Catholic Archdiocese of St Andrews and Edinburgh, means: "The Cross our only hope."

There is a slightly longer form "Ave Crux Spes Unica", which is used as a pious expression as well as being a motto.  This translates as: "Hail the Cross our only hope."  There is a short article on Wikipedia about Ave_crux_spes_unica.

Tecum habita

I saw this Latin motto tecum habita above a closed doorway in the City of Edinburgh's Central Library today.  Looking it up on the net, I find that it is the start of an epigram from the satires of Aulus Persius Flaccus.  The full epigram is: Tecum habita: noris quam sit tibi curta supellex.  [Satire IV, line 52]  In
English, it reads: "Live with yourself: get to know how poorly furnished you are."

To find out more about Persius visit Wikipedia and to read some of his other epigrams visit Wikiquote.

Nisi dominus frustra

Nisi dominus frustra is the motto of the City of Edinburgh.  It means unless the Lord in vain, which is a contraction of 127th Psalm:

Except the Lord build the house, They labour in vain that build it
Except the Lord keep the city, The watchman waketh but in vain.

Nisi is a conjunction meaning unless,  dominus, -i means lord, master, and frustra is an adverb meaning in vain.