Don't override the frames.
then where do I insert the beginning text? I cant insert text on page 1 in the boxes I created on Master A
maybe starting from scratch... how do I with 3 different text boxes on page 1 flow with different information to 3 text boxes on page 2,3,4...
how can I pdf import 3 files (107pgs each) on top of each other in ID?
Are you Placing the text from an external file, or pasting it from the clipboard?
If Placing, move the loaded cursor inside one of the master frames on the first page, hold down the Shift key and click, being sure the cursor is not on any sort of guide. The entire text file will flow using the master text frames, and ID will add all the require pages to place all the text.
If pasting, you can either override the frame on the first page only, or make a temporary first frame on the pasteboard. Paste into this frame, which will result in a red plus sign in the bottom right corner of the text frame indicating overset text. Switch to the Selection tool (black arrow) and click on the plus sign to pick up the overset text. Hover the new loaded cursor over the frame on the second page, or over the first page as above if you used a temporary frame, hodl the Shift key and click. Delete the temporary frame, if you used one, and the text will reflow to start in the first real frame, but you may have to pick up overset again in the last frame due to the reflow.