x3270 terminal emulator uses the old X11 core font system which doesn't allow to use anti-aliased (smooth) fonts like .otf, .ttf and so forth. The switch to the modern Xft (X FreeType, which supports anti-aliased fonts) font system is recommended for all X11 applications.

x3270.freetypeFont resource should be used to specify a FreeType font name in the Xft format:

<family>-<size>:<name>=<value>...

Example: "monospace-12". If you don't specify x3270.freetypeFont then x3270 behaves as usual.

This patch should be applied to the 3.3.15ga4 version of x3270 using the following commands:

cd x3270-3.3 && patch -p1 <x3270.patch
./configure
make