Inserting columns into a large table of data and converting it into a {x,y,z} list

Question

I want to convert a list of data from

{{x11,y11,x21,y21,x31,y31,...xi1,yi1},{x12,y12,x22,y22,x32,y32,...xi2yi2},...{x1j,y1j,x2j,y2j,x3j,y3j,...xij,yij}}

to

{{z,x11,y11},{z,x12,y12},{z,x13,y13},...,{z,x1j,y1j},{z+10,x21,y21},{z+10,x22,y22},{z+10,x23,y23},...,{z+10,x2j,y2j},{z+20,x31,y31}...{z+10i,xij,yij}}

So this is actually a large table of data with xy values in each two successive columns. I want to insert z values in the first column and after each two columns and then convert it into a list of xyz values in order to plot it using ListPlot.
Here is a part of the table:

data={{290, 218.2, 310, 224.6, 335, 180.1}, {291, 138.4, 311, 161.2, 336, 
  129.5}, {292, 99.61, 312, 131.5, 337, 102.4}, {293, 94.15, 313, 
  129.8, 338, 95.39}, {294, 93.47, 314, 137.2, 339, 92.25}, {295, 
  95.82, 315, 144.2, 340, 89.9}, {296, 99.15, 316, 151.4, 341, 
  90.57}, {297, 104.9, 317, 161.9, 342, 91.05}, {298, 109.5, 318, 
  173.8, 343, 92.26}, {299, 114.5, 319, 188, 344, 95.52}};

Which I manually converted to the following by inserting z values of 280 for for first xy column, 295 for next xy column and 320 for the third xy column.

dataxyz={{280, 290, 218.2}, {280, 291, 138.4}, {280, 292, 99.61}, {280, 293, 
  94.15}, {280, 294, 93.47}, {280, 295, 95.82}, {280, 296, 
  99.15}, {280, 297, 104.9}, {280, 298, 109.5}, {280, 299, 
  114.5}, {295, 310, 224.6}, {295, 311, 161.2}, {295, 312, 
  131.5}, {295, 313, 129.8}, {295, 314, 137.2}, {295, 315, 
  144.2}, {295, 316, 151.4}, {295, 317, 161.9}, {295, 318, 
  173.8}, {295, 319, 188}, {320, 335, 180.1}, {320, 336, 129.5}, {320,
   337, 102.4}, {320, 338, 95.39}, {320, 339, 92.25}, {320, 340, 
  89.9}, {320, 341, 90.57}, {320, 342, 91.05}, {320, 343, 
  92.26}, {320, 344, 95.5}}

There should be some easy ways to do that, but I could not find it. Can anyone please help?
I was also thinking about extracting the data from the table using loops. Is it possible to pull the data from the table using something like that:

For[z=280,z<600,z=z+10,
 For[i=1,i<30,i=i+2,
  For[j=1,j<300,j++,
make a list of {z,x,y} from data[[j,i]]]]]

I'm not good with programming thought. Here is the small part of the plot I am trying to make:
This whole thing can be done using spreadsheet, which I generally do, but it would be very nice if I can manipulate the list in mathematica itself. Thank you.

Answer 1

Table[
  MapIndexed[Prepend[ #[[2 i - 1 ;; 2 i]] , {280, 295, 320}[[i]]] &, 
   data ], {i, 1, 3}] // Join

I could not follow what pattern you want for z ( you are counting by 10 in the For loop example.. ? )

Here is a somewhat cleaner method:

Flatten[
  Partition[Riffle[#, {280, 295, 320}, {1, 7, 3}], 3] & /@ data,
  1]

note the output here is ordered differently, but for your plot that should not matter.