try to drag window3 above window1 in the navigator
Go to property pallette of Module and change first navigation block to the block attached with window3.
You have to keep in mind that Forms is hierarchical - meaning that Forms will navigate to the first object listed in the Object Navigator by default. If you want to change this default navigation you can either re-arrange the order of the objects - as others have suggested - or you can write code to hide the default canvas/window and show the canvas/window you want displayed. Personally, I prefer rearranging the objects. In your case, you want to rearrange the order of your "Data BLOCKS" not the order of your Window or Canvas objects. The First Block and First Item in the block is the default block/item displayed. Therefore, if you want Window3 displayed first, rearrange your data blocks so that the data block that supports Window3 is the first block in the Data Blocks node.