Is there any Adobe Flash Alternative which runs Actionscript 3 on Browser?

You can still install Adobe Flash Player (warning - it is EOL and insecure) using some archive links and hacks:

cd ~/Downloads
wget -c https://launchpad.net/~ubuntu-mozilla-security/+archive/ubuntu/ppa/+build/20380719/+files/flashplugin-installer_32.0.0.465ubuntu0.20.04.1_amd64.deb
sudo apt-get install ./flashplugin-installer_32.0.0.465ubuntu0.20.04.1_amd64.deb

sudo wget -c http://mx-linux-packages.mirror.globo.tech/mx/repo/pool/non-free/a/adobe-flashplugin/adobe-flashplugin_20200311.1.orig.tar.gz -O /var/cache/flashplugin-installer/adobe-flashplugin.tar.gz

sudo /usr/lib/flashplugin-installer/install_plugin adobe-flashplugin.tar.gz

then pin its version to prevent upgrades by using single long command:

cat <<EOF | sudo tee /etc/apt/preferences.d/pin-flash
Package: flashplugin-installer
Pin: version 32.0.0.465ubuntu0.20.04.1
Pin-Priority: 1337
EOF

Then install

• local old insecure version of Waterfox (Firefox fork) browser:

  cd ~/Downloads
  wget -c https://cdn.waterfox.net/releases/linux64/installer/waterfox-G3.2.3.en-US.linux-x86_64.tar.bz2
  tar -xf waterfox-G3.2.3.en-US.linux-x86_64.tar.bz2
  sudo chown -R root:root waterfox/ # prevent auto-update (optionally)
  cd waterfox/
  
  ./waterfox https://vem-iitg.vlabs.ac.in/Experiment_DC.html
  

• local old insecure version of Firefox ESR 52.9.0 from FTP:

  cd ~/Downloads
  wget -c https://ftp.mozilla.org/pub/firefox/releases/52.9.0esr/linux-x86_64/en-US/firefox-52.9.0esr.tar.bz2
  tar -xf firefox-52.9.0esr.tar.bz2
  sudo chown -R root:root firefox/ # prevent auto-update (optionally)
  cd firefox/
  
  ./firefox https://vem-iitg.vlabs.ac.in/Experiment_DC.html
  

and enjoy.