بس isn't standard, it's used in dialect only.
فقط and مجرد are standard Arabic.
Only=فقط
Just=مجرد
بس=only/just
Examples:
أعمل يوم الجمعة فقط (standard Arabic)
=بشتغل يوم الجمعة بس (Egyptian dialect)
=I work on Friday only.
أتكلم اللغة العربية فقط (standard Arabic)
=بتكلم عربي بس (Egyptian dialect)
=I speak Arabic only.
هذا مجرد سؤال (standard Arabic)
=ده مجرد سؤال (Egyptian dialect)
=This is just a question.