You can actually build Electrum from the source yourself with the last commits, since AFAIK Segwit is already somehow usable. But I wouldn't recommend. Just follow the GitHub repo[1] README.
[1] https://github.com/spesmilo/electrum
On that page under "Development version", I found the following series of what I believe to be Linux commands:
git clone git://github.com/spesmilo/electrum.git
cd electrum
python3 setup.py install
sudo apt-get install pyqt5-dev-tools
pyrcc5 icons.qrc -o gui/qt/icons_rc.py
sudo apt-get install protobuf-compiler
protoc --proto_path=lib/ --python_out=lib/ lib/paymentrequest.proto
sudo apt-get install python-pycurl gettext
./contrib/make_locale
If I create an Electrum folder in the persistent storage of my Tails USB, open a terminal in that folder, and enter those commands, will I theoretically end up with a functioning copy of Electrum v.3?