Ok to take the pain away, I wrote this plus more, as library routines to do what your trying to do, it only takes a couple of lines of code to work.
(if (not AH:getvalsm)(load "Multi Getvals.lsp"))
(setq ans (AH:getvalsm (list "Enter values " "Lengte " 5 4 "100" "Breedte " 5 4 "100" "Hoogte" 5 4 "100")))
(setq lengte (atof (nth 0 ans)) Breedte (atof (nth 1 ans)) Hoogte (atof (nth 2 ans)))
You just need to save Multi getvals.lsp ina support path or change the (load "D:\\fullpath name\\Multi getvals") to include full path use "\\" for directories.

A lot of us don't write a separate dcl but rather have the make a dcl as part of the lisp, using a write temp file look at attached.
Back to yours redid the dcl
widhtlen : dialog {
label = "Please enter values" ;
: column {
width = 25 ;
: edit_box { //define edit box
key = "Len" ; //give it a label
label = "Lengte :" ; //give it a lenght
mnemonic = "";
edit_width = 20 ; //30 characters
} //end edit box
//end edit box
: edit_box { //define edit box
key = "bred" ; //give it a label
label = "Breedte :" ; //give it a width
mnemonic = "";
edit_width = 20 ; //30 characters
} //end edit box
//end edit box
: edit_box {
key = "hogt" ;
label = "Hoogte :" ;
mnemonic = "";
edit_width = 20 ;
}
}
spacer_1 ;
ok_cancel ; }
(defun c:lenwidht ( / )
(setq dcl_id (load_dialog "d:\\acadtemp\\temp.dcl"))
(if (not (new_dialog "widhtlen" dcl_id))
(exit)
)
(mode_tile "Len" 2)
(action_tile "len" "(setq Lengte (atof $value))")
(action_tile "bred" "(setq Breedte(atof $value))")
(action_tile "hogt" "(setq Hoogte (atof $value))")
(action_tile "accept" "(done_dialog)")
(action_tile "cancel" "(done_dialog)")
(start_dialog)
(unload_dialog dcl_id)
(princ)
)