When I select Line x3 mode, my TV scale to 1280x720p/50p : is it normal behavior ?
That figure is not what your TV is scaling to, it is the input resolution (i.e. the output from the OSSC). From your pictures, we see V.Active = 240, and 240×3 = 720 so it makes sense.
I notice your image is also cropped vertically, so you could test other V.active, like 256 for 1280×768 output, or do Lx4 1080p (V. active is then 270). See the stickied tips & tweaks thread.
Why your image is cropped horizontally is a bit weird. Looks like it crops 1 pixel each side (there should be an outer red border on the grid pattern, if I’m not mistaken the squares in the grid are 16×16 pixels, 16 across and 14 tall so 256×224). Then it would seem you should set h.active to 258, though I find that odd, afaik PAL NES does not output wider visible area than NTSC. Additionally it could cause the image to drop because then you have less standard image frame (1290×720 instead of 1280×720). Also I see you have sample rate at 341.1, that fractional samplerate might affect the width (I’m also not sure it’s needed for optimal sampling, 341 should work fine).
Finally make sure any overscan (zoom) setting on your TV is disabled.
Anyways, some things to try.