Answer:I’ll assume you mean what all three suffixes have in common: signaling nouns that mean “a person who” (instead of, for example, ‑er to signal a comparative adjective, or ‑or as a spelling of abstract nouns formed from -id adjectives derived from Latin).
In this sense, the ‑ist suffix differs from ‑er and ‑or in the following ways:
The suffix ‑ist is usually attached to a root or adjective, whereas the suffixes ‑er and ‑or are generally attached to a whole-word verb or else (in the case of ‑or) remain unchanged from their original Latin form.
The meaning is more that of “adherent” or “professional practitioner” of the root than “one who does” the root.
Many ‑ist words have an explicit or implicit ‑ism ideology or movement.
Some ‑ist words arise from an explicit or implicit ‑ize/‑ise verb.
Some ‑ist words derive directly from a French ‑iste form; some ‑er nouns derive directly from a French ‑eur form (but most are living suffixes appended to English verbs); some ‑or nouns derive directly from the Latin ‑or form.
For example:
specialist —
(1) There is no verb “to special”;
(2) a specialist is a professional practitioner;
(4) an associated verb is specialize/specialise, but the term specialist is specific to a medical specialty.
Communist —
(2) A Communist is an adherent;
(3) Communism is an ideology / movement.
cellist —
(1) There is no verb “to cell” or “to cello”;
(2) A cellist is a professional practitioner.
Contrast:
adviser, caller, preacher, speaker, striker, teacher:
(1) There are verbs advise, call, preach, speak, strike, teach;
(2) They mean “one who <verb>s” (or “that which <verb>s”) more than they mean an adherent or professional practitioner of <root>;
(3) There is no ideology advism, callism, preachism, etc.
(4) Except for advise, there are no verbs callize, preachise, etc.
advisor, actor, inspector, investor, sailor
(1) There are verbs advise, act, inspect, invest, and sail;
(2) They mean “one who <verb>s” or “that which <verb>s”) more than they mean an adherent or professional practitioner of <root>;
(3) There is no ideology advism, actism, inspectism, etc.
(4) Except for advise, there are no verbs actise, …, sailize.
conductor, creditor, debtor, malefactor
(1,5) They remain mostly unchanged from the original Latin;
(2) They mean “one who <verb>s” or “that which <verb>s”) more than they mean an adherent or professional practitioner of <root>;
(3) There is no ideology conductism, creditism, debtism, etc.
(4) There are no verbs conductize, creditize, …, malefactise.
Explanation:
