job.vim

function s:handler_close_cb(channel) abort
    let lines = []
    while ch_status(a:channel, {'part': 'out'}) == 'buffered'
        let lines += [ch_read(a:channel)]
    endwhile
    echo lines
endfunction

function s:main() abort
    let src = expand('temp.vim')
    call writefile(['echo "1"', 'echo "2"', 'echo "3"'], src)
    let cmd = ['vim', '-X', '-N', '-u', 'NONE', '-i', 'NONE', '-V1', '-e', '-s', '-S', src, '+qall!']
    let job = job_start(cmd, {
            \ 'close_cb' : function('s:handler_close_cb'),
            \ 'out_io' : 'pipe',
            \ 'err_io' : 'out',
            \ })
endfunction

call s:main()
" ['1', '2', '3
" 3
" 3']

このままだと、すぐechoが消えちゃうので、echomsgとかに残すほうがいいかも