Putting out an SOS on an frustrating issue i'm having with the UI Template & positioning my groups.
Here's the flow:
[{"id":"3e588bb4.6427e4","type":"ui_template","z":"37005b7e.7f759c","group":"a59619e6.127c48","name":"O-Box","order":1,"width":5,"height":9,"format":"<!DOCTYPE html>\n<html>\n\n<style>\n#Test {\n display: flex;\n float: center;\n width: 1920px;\n}\n#Test_Panel1 {\n display: flex;\n float: center;\n height: 502px;\n width: 813px;\n border: 1px solid #ffffff;\n padding: 10px;\n position: static;\n margin-top: 5px;\n margin-left: 5px;\n margin-right: 5px;\n margin-bottom: 5px;\n}\n</style>\n\n</html>","storeOutMessages":true,"fwdInMessages":false,"resendOnRefresh":true,"templateScope":"local","x":1190,"y":640,"wires":[[]],"icon":"node-red-dashboard/ui_switch.png"},{"id":"e05b1101.e0b33","type":"ui_template","z":"37005b7e.7f759c","group":"e2468ce4.2c341","name":"I-Box","order":1,"width":16,"height":8,"format":"<!DOCTYPE html>\n<html>\n \n<style>\n#Test_Panel2 {\n display: flex;\n float: left;\n height: 454px;\n width: 813px;\n border: 1px solid #ffffff;\n padding: 10px;\n position: static;\n margin-top: 0px;\n margin-left: 5px;\n margin-right: 5px;\n margin-bottom: 5px;\n}\n</style>\n\n</html>","storeOutMessages":true,"fwdInMessages":false,"resendOnRefresh":true,"templateScope":"local","x":1190,"y":680,"wires":[[]],"icon":"font-awesome/fa-info-circle"},{"id":"c34dc90f.e90a08","type":"ui_template","z":"37005b7e.7f759c","group":"10edc74b.d84669","name":"W-Box","order":2,"width":20,"height":"17","format":"<!DOCTYPE html>\n<html>\n\n<style>\n#Test_Panel3 {\n \n float: right;\n height: 983px;\n width: 1038px;\n border: 1px solid #ffffff;\n padding: 10px;\n position: static;\n margin-top: 5px;\n margin-left: 0px;\n margin-right: 5px;\n margin-bottom: 5px;\n}\n</style>\n\n</html>","storeOutMessages":true,"fwdInMessages":false,"resendOnRefresh":true,"templateScope":"local","x":1190,"y":720,"wires":[[]],"inputLabels":["B64 Format"],"icon":"font-awesome/fa-file-image-o"},{"id":"a59619e6.127c48","type":"ui_group","z":"","name":"Panel1","tab":"32694a66.f815a6","order":1,"disp":false,"width":"16","collapse":false},{"id":"e2468ce4.2c341","type":"ui_group","z":"","name":"Panel2","tab":"32694a66.f815a6","order":2,"disp":false,"width":"16","collapse":false},{"id":"10edc74b.d84669","type":"ui_group","z":"","name":"Panel3","tab":"32694a66.f815a6","order":3,"disp":false,"width":"20","collapse":false},{"id":"32694a66.f815a6","type":"ui_tab","z":"","name":"Test","icon":"dashboard","disabled":false,"hidden":false}]
All i want to do is have all three group elements on ONE page & can't understand why I can't fit the 3rd panel in(WITHOUT having a page-sized gap/margin at the bottom). I've tried using different combo's of 'display', 'position' & 'float' & have had them aligned, when i using 'minus' margin values, but ALWAYS extends the page(even though the width/height is within the page threshold!?).
I've read MANY articles, gone through tutorials that work on the browser...but not when in Node-Red!? PLEASE any help as to WHY this "simple" task seems so taxing, its draining my will to live.... 
I'm going to experiment with the UI Builder after this project, so thanks for the suggestion, hotNipi. Colin's method will suffice for this problem. I've overcome more complex issues with scripts contained within these groups...think i was overthinking the issue & couldn't see the wood for the trees, so I thank you both for bringing me back to ground level. 