only last dynamically created picturebox is accessible

try this:

 public partial class Form1 : Form
    {
        PictureBox[] picturebox1 = new PictureBox[3];//array of three pictureBoxes
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            disablebutts(false);
        }

        public void disablebutts(bool cat)// disable buttons so user cannot click before pictureboxes are created
        {
            button2.Enabled = cat;
            button3.Enabled = cat;
            button4.Enabled = cat;
        }
        private void button1_Click(object sender, EventArgs e)
        {
            disablebutts(true);
            TabPage tpgallery = new TabPage();
            tpgallery.Name = "tpgallery";
            tpgallery.Text = "  Gallery  ";

            tabControl1.TabPages.Add(tpgallery);
            int lft = 0;
            for (int i = 0; i < 3; i++)//create the pictureBoxes
            {
                picturebox1[i] = new PictureBox();
                picturebox1[i].Name = ("picturebox" + i.ToString());
                picturebox1[i].SizeMode = PictureBoxSizeMode.AutoSize;
                picturebox1[i].Image = Properties.Resources.logo1;
                picturebox1[i].BorderStyle = BorderStyle.FixedSingle;
                picturebox1[i].Left = lft;//set boxes 200 apart
                //if (i == 2)//use to place elsewhere
                //{
                //    tabPage1.Controls.Add(picturebox1[i]);
                //}
                //else
                //{
                //    tpgallery.Controls.Add(picturebox1[i]);
                //}
                tpgallery.Controls.Add(picturebox1[i]);
                lft += 200;
            }

            
        }

        private void button2_Click(object sender, EventArgs e)
        {
            picturebox1[0].Image = Properties.Resources.logo2;//change image on first picturebox
            picturebox1[0].Refresh();      

        }

        private void button3_Click(object sender, EventArgs e)
        {
            picturebox1[1].Image = Properties.Resources.logo2;//change image on second picturebox
            picturebox1[1].Refresh();
        }

     
        private void button4_Click(object sender, EventArgs e)
        {
            picturebox1[2].Image = Properties.Resources.logo2;//change image on third picturebox
            picturebox1[2].Refresh();
        }
    }