Download development releases of modules with Drush Make
I tried generating a Drush make file with http://drushmake.me/, and the generated file contained the following lines.

projects[jquery_ui][version] = 1.x-dev
projects[jquery_ui][type] = "module"
projects[libraries][version] = 2.1
projects[libraries][type] = "module"
projects[xmlsitemap][version] = 2.0-rc2
projects[xmlsitemap][type] = "module"


I think that in your case you should use the following lines.

projects[better_exposed_filters][version] = 3.x-dev
projects[better_exposed_filters][type] = "module"


That would select the development snapshot for 3.x; in the case you want to select the 7.x-3.0-beta3 version, then you should use the following lines.

projects[better_exposed_filters][version] = 3.0-beta3
projects[better_exposed_filters][type] = "module"


The make file needs to contain the following line, which select Drupal 7 as Drupal version to use. This alters the version used for the modules; for example, the following line used with the latter lines I shown, would pick-up version 7.3.0-beta3 of the Better Exposed Filters module.

projects[drupal][version] = 7