If you aim for something that works similar to Semitic languages, then the patterns (what you call "stems", like CeCïCik; "stem" normally refers to the combination of root and derivational morphology, but no inflectional morphology) should be what in other languages are derivational and inflectional morphology, while the roots are what in other languages are roots.

In Semitic languages, the patterns usually encode agreement with subject in person, number, and gender, grammatical aspect, mood, voice, number, definiteness, case, and state for nouns, and possessive pronouns on nouns and prepositions, derivational concepts like "place for X" and "tool for X", etc.

Other natural languages have other concepts encoded in the morphology. Anything like that may be a pattern in a triliteral language. There's "gather/hunt for X", "have X", etc.

I think modern Semitic languages have acquired patterns for modern concepts like "software", so that Hebrew or Arabic could take the root k-t-b "write" and combine it with this "software" pattern, and get a word that means "word processor".

Anything that would be useful as part of a compound in any other language might be a good pattern in a triliteral language.

Languages may have an affix for "language", similar to English "-ish" in "English". I think Hebrew uses the suffix -it as such an affix, rather than a pattern for it. Thus you get "yehudit" "Jewish language" from "yehuda" "Jew", and "'ivrit" "Hebrew language" from "'ever" "Hebrew person".

But there are roots for "say" and such in Semitic languages. "say" in Hebrew is the root ʔ-m-r, and in Arabic q-w-l.

Thus a root Sh-K-T for "speak" might be more natural than a pattern CeCïCik.

In Semitic languages, you don't just use patterns combined with roots, but also prefixes and suffixes. When affixes are added to a word, it might change internally, but not that much that it is a completely different pattern, just adjusted a little. E.g. "'ever" mentioned above becomes "'ivr" when the suffix "it" is added to it. Much of the "patterns" are like that, they are not completely unrelated patterns, but combinations of affixes, with accompanying adjustments to the stem.

In Biblical Hebrew, subject agreement and aspect are indicated with prefixes, suffixes and circumfixes, while the pattern of the stem they are added to indicate voice and aspect. Also, nouns in Hebrew use suffixes to mark number, where the stem might adjust a little, but is essentially the same.

I don't speak any Semitic language, so details in this post might be wrong.

But it you don't aim at something that works like Semitic languages, then you could do anything.