gdal_merge.py -createonly -separate -co PHOTOMETRIC=RGB -o merged.tif LE07_L1TP_046030_20180810_20180810_01_RT_b3.tif LE07_L1TP_046030_20180810_20180810_01_RT_b2.tif LE07_L1TP_046030_20180810_20180810_01_RT_b1.tif

gdalwarp --config GDAL_CACHEMAX 512 -wm 4096 -nomd LE07_L1TP_046030_20180810_20180810_01_RT_b3.tif LE07_L1TP_046030_20180810_20180810_01_RT_b2.tif  LE07_L1TP_046030_20180810_20180810_01_RT_b1.tif  merged.tif

Size is 9930, 9315
Coordinate System is:
PROJCS["Albers",
    GEOGCS["NAD83",
        DATUM["North_American_Datum_1983",
            SPHEROID["GRS 1980",6378137,298.2572221010042,
                AUTHORITY["EPSG","7019"]],
            AUTHORITY["EPSG","6269"]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433],
        AUTHORITY["EPSG","4269"]],
    PROJECTION["Albers_Conic_Equal_Area"],
    PARAMETER["standard_parallel_1",29.5],
    PARAMETER["standard_parallel_2",45.5],
    PARAMETER["latitude_of_center",23],
    PARAMETER["longitude_of_center",-96],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]]]
Origin = (-2336089.094018519900000,2701776.184199890100000)
Pixel Size = (30.000000000000000,-30.000000000000000)
Metadata:
  AREA_OR_POINT=Area
Image Structure Metadata:
  INTERLEAVE=PIXEL
Corner Coordinates:
Upper Left  (-2336089.094, 2701776.184) (125d42'39.19"W, 43d59'36.72"N)
Lower Left  (-2336089.094, 2422326.184) (124d40'18.81"W, 41d36'34.34"N)
Upper Right (-2038189.094, 2701776.184) (122d 7'21.46"W, 44d46' 3.96"N)
Lower Right (-2038189.094, 2422326.184) (121d11'45.22"W, 42d21'15.67"N)
Center      (-2187139.094, 2562051.184) (123d25'35.42"W, 43d11'34.14"N)
Band 1 Block=9930x1 Type=Byte, ColorInterp=Red
    Computed Min/Max=0.000,255.000
  Minimum=0.000, Maximum=255.000, Mean=27.268, StdDev=43.782
  Metadata:
    STATISTICS_MAXIMUM=255
    STATISTICS_MEAN=27.267951614063
    STATISTICS_MINIMUM=0
    STATISTICS_STDDEV=43.781663272236
Band 2 Block=9930x1 Type=Byte, ColorInterp=Green
    Computed Min/Max=0.000,0.000
  Minimum=0.000, Maximum=0.000, Mean=0.000, StdDev=0.000
  Metadata:
    STATISTICS_MAXIMUM=0
    STATISTICS_MEAN=0
    STATISTICS_MINIMUM=0
    STATISTICS_STDDEV=0
Band 3 Block=9930x1 Type=Byte, ColorInterp=Blue
    Computed Min/Max=0.000,0.000
  Minimum=0.000, Maximum=0.000, Mean=0.000, StdDev=0.000
  Metadata:
    STATISTICS_MAXIMUM=0
    STATISTICS_MEAN=0
    STATISTICS_MINIMUM=0
    STATISTICS_STDDEV=0

gdal_merge.py -createonly -separate -co PHOTOMETRIC=RGB -o merged.tif redband.tif greenband.tif blueband.tif

gdalbuildvrt -separate merged.vrt redband.tif greenband.tif blueband.tif

gdal_translate merged.vrt merged.tif