Trying to Install libcurl3 and libcurl4

Question

I am trying to follow this guide to use libcurl3 and 4 but am unable to install libcurl3 with apt as it is unavailable.

root@raspberrypi:/home/pi/tss/tsschecker-master/git/tsschecker# apt install libcurl3
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package libcurl3 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
  libcurl4

E: Package 'libcurl3' has no installation candidate