So from the Sentinel 5P image collection I created 3 different images for 3 years (mean of - 2019, 2020 and 2021). Then I clipped those 3 images using a Geometry and then again 3 images were made. Now I want to combine these 3 images into a single one so that while extracting data from that combined image I will be able to get data for the 3 years (2019, 2020 and 2021). I tried this method -

var simpleJoin = ee.Join.simple();
var mod1join = ee.ImageCollection(simpleJoin.apply(img1clip, img2clip, img3clip));
Map.addLayer(mod1join, band_viz);

But while loading the layer it gives me an error -

Layer 1: Layer error: Join.apply, argument 'secondary': Invalid type. Expected type: FeatureCollection. Actual type: Image<[SO2_column_number_density]>.

I tried searching for this error but did not find any solution. What will be the solution to combine the 3 images of different years, keeping the data for those particular years as well?

Below I am attaching the code of what I did and tried -

var img1 = ee.ImageCollection(imageCollection
.select('SO2_column_number_density')
.filterBounds(geometry)
.filterDate('2019-01-01', '2019-12-31'))
//remove the negative values from the band
//.map(function(image){return image.updateMask(image.gte(0))});
print('no. of img1', img1.size());
var img2 = ee.ImageCollection(imageCollection
.select('SO2_column_number_density')
.filterBounds(geometry)
.filterDate('2020-01-01', '2020-12-31'))
print('no. of img2', img2.size());
var img3 = ee.ImageCollection(imageCollection
.select('SO2_column_number_density')
.filterBounds(geometry)
.filterDate('2021-01-01', '2021-12-31'))
print('no. of img3', img3.size());
var band_viz = {
  min: 0.0,
  max: 0.0005,
  palette: ['black', 'blue', 'purple', 'cyan', 'green', 'yellow', 'red']
};
var img1map = img1.mean();
var img2map = img2.mean();
var img3map = img3.mean();

//Map.addLayer (SP5map, band_viz);
var img1clip = img1map.clip(geometry);
var img2clip = img2map.clip(geometry);
var img3clip = img3map.clip(geometry);

//print(img1clip);

var simpleJoin = ee.Join.simple();
var mod1join = ee.ImageCollection(simpleJoin.apply(img1clip, img2clip, img3clip));
Map.addLayer(mod1join, band_viz);

FYI: All the 3 clipped images contain only 1 band.