How many watts during grow

Does anyone have any recommendations on how many watts during the grow cycle I have a 4x8 with 3 led lights totaling 900 watts I think a good number but my question is more so on dimming the lights should I keeps the plants condensed under 1 or 2 lights untill the room fills out and should the lights be on full blast or should I dim the lights and bring them up as the room fills out

Best way to know is PPFD maps and DLI. Are the three lights the same? What are they? Do you have PPFD maps for them?
If it is 900 draw watts it sounds a little lite unless you are not using all the space for flower.
I have 450 in a 3x3, in a 2x2 I dim my 400 down to 320, in my 32"x72" closet I have three HLG 260’s that I dim down to 660 from 750.

Agree with above, if possible look for photone app, it free, need a piece of paper will help get your DLI dialed in. Good luck

This what chart I have been following
Download the Photone app on your phone and make sure you use a defuser(piece of white copy paper) ti place over your camera lens and your off to the races

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They are all Mars hydro 300 watt I can fit another if it’s needed

I use a 300 watt i basically keep it on flat chat but i have my light up higher i dont so the whole 18inches or the 24 inches or the 32 inch thing i do things alot differently to other growers

The DLI graph from @Sincitytoker tells us that during flower on a 12 hour light schedule we need DLI of around 40. (Many around here shoot for more). A DLI of 40 requires an average PPFD of 940. A DLI of 35 requires an average PPFD of 820.

820 3600 12 1000000 35.424

I push just a little higher than the red line

50 DLI is right around 1160 PPFD for 12 hours. Approaching the range when CO2 is needed

At least based on what LumiGrow shows in this chart.
At any rate that is a lot of light. Which, I know you have given your combo of HLG 260XL with HLG135