Bash - cat command and errors

Question

I was trying bash shell redirection. I have a directory where there is just one file - a.txt

more a.txt
HELLO

If I type cat < a.txt b.txt , I get

cat: b.txt: No such file or directory

If I type

cat  < a.txt   b.txt a.txt

I get

cat: b.txt: No such file or directory
HELLO

Why am I not seeing HELLO when I try cat < a.txt b.txt? I tried different combinations - typing a.txt multiple times before/after b.txt and see a pattern, but not able to figure out why.

cat  <   a.txt a.txt b.txt  
HELLO
cat: b.txt: No such file or directory