Plot3D[Min[x, y], {x, 0, 20}, {y, 0, 23}]

Plot3D@Min@x, yD, 8x, 0, 20<, 8y, 0, 23<D
Plot3D@Min@3 x, yD, 8x, 0, 20<, 8y, 0, 23<D
Plot3D@Min@x, 3 yD, 8x, 0, 20<, 8y, 0, 23<D
Plot3D@Min@3 x, 3 yD, 8x, 0, 20<, 8y, 0, 23<D
20
15
10
5
0
0
20
15
10
5
10
5
15
20 0
SurfaceGraphics
20
15
10
5
0
0
20
15
10
5
10
5
15
20 0
SurfaceGraphics
20
15
10
5
0
0
20
15
10
5
10
5
15
20 0
SurfaceGraphics
60
40
20
20
15
0
0
10
5
10
5
15
20 0
SurfaceGraphics
ContourPlot@Min@x, yD, 8x, 0, 20<, 8y, 0, 23<D
ContourPlot@Min@3 x, yD, 8x, 0, 20<, 8y, 0, 23<D
ContourPlot@Min@x, 3 yD, 8x, 0, 20<, 8y, 0, 23<D
ContourPlot@Min@3 x, 3 yD, 8x, 0, 20<, 8y, 0, 23<D
20
15
10
5
0
0
5
ContourGraphics
10
15
20
20
15
10
5
0
0
5
10
15
20
10
15
20
ContourGraphics
20
15
10
5
0
0
5
ContourGraphics
20
15
10
5
0
0
5
ContourGraphics
10
15
20