The vars t1 - t3 are inputs of each implicit function, t4 - t6 are outputs.
I need to find the region of t1 - t3 in which t4 - t6 can be found in Real filed.
Here I proposed two methods. The first one does work, and the regions can be plotted separately, but I can't intersect these 3 regions. The second one doesn't work.
Method 1

region1 = 
 RegionPlot3D[
  NSolve[equ1 == 0, t4, Reals] != {}, {t1, -Pi/2, Pi/2}, {t2, -Pi, 
   0}, {t3, -Pi/2, Pi/2}]

Method 2

R = ImplicitRegion[equ1 == 0, {t1, t2, t3, t4}]
prj = Resolve[Exists[{t4}, Element[{t1, t2, t3, t4}, R]], Reals]; 
RegionPlot3D[prj, {t1, -(Pi/2), Pi/2}, {t2, -Pi, 0}, {t3, -(Pi/2), Pi/2}]

THANK YOU !