controller.rb

class EnrollmentController < ApplicationController
  def new
    @users_assignments = []
    3.times{ @users_assignments << @event.assignments.build }
  end
end

form.html.erb

<%= semantic_form_for @event do |f| %>
  
  <!--
    Usually, there would be semantic_fields_for :jobs here, 
    but the user choses the job in a wishlist-form.
    
    This leads to a
      ActiveRecord::HasManyThroughCantAssociateThroughHasOneOrManyReflection error, saying
      "Cannot modify association 'Event#assignments' because the source reflection class 'Assignment' 
      is associated to 'Job' via :has_many"
  -->

  <%= f.semantic_fields_for :assignments, @assignments do |a| %>
    <%= a.input :job, as: :select, collection: @users_assignments %>
  <% end %>
<% end %>

models.rb

class Event < ActiveRecord::Base
  has_many :jobs
  has_many :assignments, through: :jobs
end

class Job < ActiveRecord::Base
  belongs_to :event
  has_many :assignments
end

class Assignment
  belongs_to :job
end