You aren’t modernish just Farmished if you say Seuda Shlishis. Its very rare to hear a Seuda Shlishi”s”. Its either Seuda Shlishit or Shalosh Seudot. Or Shahloshudis. Or Shaluhsheedis or Shalsheedis if you are Chassidish.
In my house you’d probably hear them all.
My daughter went to a school where the modern, Israeli style pronunciation is used. My boys have a more yeshivish style (including using “oy” for a cholom — which ALWAYS grates on my nerves). I use an Ashkenazic pronunciation.
The Wolf