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Yes, please check this guide: http://www.jotform.com/help/35-The-Importance-of-Form-Widths. For field positioning, check this guide: Form Field Positioning.
It is definitely possible to increase the width of the form. You do that simply by clicking on the Form Width option and then adjusting it to your desired size
Should you want to put 3 fields across, you should use the "Shrink" option in a field's property. That way, it only takes up the space it needs and not the entire line
With the Form's width big enough, you can now shrink 3 fields to occupy one line. This is the result..