Help: Problem in using image detection filters

I have a fractography image. I want to determine the edge detection in the fractographs... I want to compare the edge detection filters in matlab( sobel, prewitt,canny).. The following code is written

close all

a = imread('3.png);

imshow(a)

figure

imedge = edge(img, 'sobel',0.05);

imshow(imedge, 'ColorMap', [1 1 1; 1 0 0]);

figure

title('sobel');

img = imread('3.png');

imedge1 = edge(img, 'prewitt',0.05);

imshow(imedge1, 'ColorMap', [1 1 1; 1 0 0]);

figure

title('prewitt');

img = imread('3.png');

imedge2 = edge(img, 'canny',0.05);

imshow(imedge2, 'ColorMap', [1 1 1; 1 0 0]);

figure

title('canny')

Ending up in the error

Error using images.internal.imageDisplayValidateParams>validateCData (line 115)

Multi-plane image inputs must be RGB images of size MxNx3.

Error in images.internal.imageDisplayValidateParams (line 27)

common_args.CData = validateCData(common_args.CData,image_type);

Error in images.internal.imageDisplayParseInputs (line 78)

common_args = images.internal.imageDisplayValidateParams(common_args);

Error in imshow (line 246)

images.internal.imageDisplayParseInputs({'Parent','Border','Reduce'},preparsed_varargin{:});

Error in ruoc (line 3)

imshow(a)

Please anyone suggest the code/command for this.. Any help is much appreciated..

Please find the attachment of the image...